Print type of data constructors correctly in GHCi

ToDo comments

A round of space-leak fixing.

  - re-instate zapping of the PersistentCompilerState at various
    points during the compilation cycle in HscMain.  This affects
    one-shot compilation only, since in this mode the information
    collected in the PCS is not required after creating the final
    interface file.

  - Unravel the recursive dependency between MkIface and
    CoreTidy/CoreToStg.  Previously the CafInfo for each binding was
    calculated by CoreToStg, and fed back into the IdInfo of the Ids
    generated by CoreTidy (an earlier pass).  MkIface then took this
    IdInfo and the bindings from CoreTidy to generate the interface;
    but it couldn't do this until *after* CoreToStg, because the CafInfo
    hadn't been calculated yet.  The result was that the CoreTidy
    output lived until after CoreToStg, and at the same time as the
    CorePrep and STG syntax, which is wasted space, not to mention
    the complexity and general ugliness in HscMain.

    So now we calculate CafInfo directly in CoreTidy.  The downside is
    that we have to predict what CorePrep is going to do to the
    bindings so we can tell what will turn into a CAF later, but it's
    no worse than before (it turned out that we were doing this
    prediction before in CoreToStg anyhow).

  - The typechecker lazilly typechecks unfoldings.  It turns out that
    this is a good idea from a performance perspective, but it also
    means that it must hang on to all the information it needs to
    do the typechecking.  Previously this meant holding on to the
    whole of the typechecker's environment, which includes all sorts
    of stuff which isn't necessary to typecheck unfoldings.  By paring
    down the environment captured by the lazy unfoldings, we can
    save quite a bit of space in the phases after typechecking.

Import pruning, use mingw32_TARGET not HOST, use old-style foreign import (for 4.08 compat)

Another datacon-naming wibble

-------------------------------------
  Big upheaval to the way that constructors are named
	-------------------------------------

This commit enshrines the new story for constructor names.  We could never
really get External Core to work nicely before, but now it does.

The story is laid out in detail in the Commentary
	ghc/docs/comm/the-beast/data-types.html
so I will not repeat it here.

	[Manuel: the commentary isn't being updated, apparently.]

However, the net effect is that in Core and in External Core, contructors look
like constructors, and the way things are printed is all consistent.

It is a fairly pervasive change (which is why it has been so long postponed),
but I hope the question is now finally closed.

All the libraries compile etc, and I've run many tests, but doubtless there will
be some dark corners.

------------------------------------
	(a) Improve reporting of staging errors
	(b) Tidy up the construction of dict funs
			and default methods
	------------------------------------

Disable the version check on the .hi file for --show-iface.  This
means that it can be used on .hi files for other "ways" (eg. profiled)
without having to resort to using the undocumented -buildtag option.

import trimming

----------------------------------------
	Class ops that do not introduce for-alls
	----------------------------------------

	MERGE TO STABLE (if poss)

The handling of class ops that do not add an extra for-all
was utterly bogus.  For example:

	class C a where
	    fc :: (?p :: String) => a;

	class D a where
	    fd :: (Ord a) => [a] -> [a]

De-bogus-ing means

a) Being careful when taking apart the class op type in
	MkIface.tcClassOpSig

b) Ditto when making the method Id in an instance binding.
   Hence new function Inst.tcInstClassOp, and its calls
   in TcInstDcls, and TcClassDcls

------------------------
	More dependency fiddling
	------------------------

	WARNING: Interface file format has changed (again)
		 You need to 'make clean' in all library code

* Orphan modules are now kept separately
  Home-package dependencies now contain only home-package dependencies!
  See HscTypes.Dependencies

* Linker now uses the dependencies to do dynamic linking

Result: Template Haskell should work even without --make (not yet tested)

Uh oh; got the versioning stuff a bit wrong in the last commit

------------------------------------------
	1. New try and module and package dependencies
	2. OrigNameCache always contains final info
	------------------------------------------

These things nearly complete sorting out the incremental
linking problem that started us off!

1. This commit separates two kinds of information:

  (a) HscTypes.Dependencies:
	What (i)  home-package modules, and
	     (ii) other packages
      this module depends on, transitively.

      That is, to link the module, it should be enough
      to link the dependent modules and packages (plus
      any C stubs etc).

      Along with this info we record whether the dependent module
      is (a) a boot interface or (b) an orphan module.  So in
      fact (i) can contain non-home-package modules, namely the
      orphan ones in other packages (sigh).

  (b) HscTypes.Usage:
      What version of imported things were used to
      actually compile the module.  This info is used for
      recompilation control only.



2. The Finder now returns a correct Module (incl package indicator)
first time, so we can install the absolutely final Name in the
OrigNameCache when we first come across an occurrence of that name,
even if it's only an occurrence in an unfolding in some other interface
file.  This is much tidier.

As a result Module.lhs is much cleaner
	No DunnoYet
	No mkVanillaModule
ALl very joyful stuff.

--------------------------------
        Implement recursive do-notation
	--------------------------------

This commit adds recursive do-notation, which Hugs has had for some time.

	mdo { x <- foo y ;
	      y <- baz x ;
	      return (y,x) }

turns into

	do { (x,y) <- mfix (\~(x,y) -> do { x <- foo y;
					    y <- baz x }) ;
	     return (y,x) }

This is all based on work by Levent Erkok and John Lanuchbury.

The really tricky bit is in the renamer (RnExpr.rnMDoStmts) where
we break things up into minimal segments.  The rest is easy, including
the type checker.

Levent laid the groundwork, and Simon finished it off. Needless to say,
I couldn't resist tidying up other stuff, so there's no guaranteed I
have not broken something.

--------------------------------------
	Make Template Haskell into the HEAD
	--------------------------------------

This massive commit transfers to the HEAD all the stuff that
Simon and Tim have been doing on Template Haskell.  The
meta-haskell-branch is no more!

WARNING: make sure that you

  * Update your links if you are using link trees.
    Some modules have been added, some have gone away.

  * Do 'make clean' in all library trees.
    The interface file format has changed, and you can
    get strange panics (sadly) if GHC tries to read old interface files:
    e.g.  ghc-5.05: panic! (the `impossible' happened, GHC version 5.05):
	  Binary.get(TyClDecl): ForeignType

  * You need to recompile the rts too; Linker.c has changed


However the libraries are almost unaltered; just a tiny change in
Base, and to the exports in Prelude.


NOTE: so far as TH itself is concerned, expression splices work
fine, but declaration splices are not complete.


		---------------
		The main change
		---------------

The main structural change: renaming and typechecking have to be
interleaved, because we can't rename stuff after a declaration splice
until after we've typechecked the stuff before (and the splice
itself).

* Combine the renamer and typecheker monads into one
	(TcRnMonad, TcRnTypes)
  These two replace TcMonad and RnMonad

* Give them a single 'driver' (TcRnDriver).  This driver
  replaces TcModule.lhs and Rename.lhs

* The haskell-src library package has a module
	Language/Haskell/THSyntax
  which defines the Haskell data type seen by the TH programmer.

* New modules:
	hsSyn/Convert.hs 	converts THSyntax -> HsSyn
	deSugar/DsMeta.hs 	converts HsSyn -> THSyntax

* New module typecheck/TcSplice type-checks Template Haskell splices.

		-------------
		Linking stuff
		-------------

* ByteCodeLink has been split into
	ByteCodeLink	(which links)
	ByteCodeAsm	(which assembles)

* New module ghci/ObjLink is the object-code linker.

* compMan/CmLink is removed entirely (was out of place)
  Ditto CmTypes (which was tiny)

* Linker.c initialises the linker when it is first used (no need to call
  initLinker any more).  Template Haskell makes it harder to know when
  and whether to initialise the linker.


	-------------------------------------
	Gathering the LIE in the type checker
	-------------------------------------

* Instead of explicitly gathering constraints in the LIE
	tcExpr :: RenamedExpr -> TcM (TypecheckedExpr, LIE)
  we now dump the constraints into a mutable varabiable carried
  by the monad, so we get
	tcExpr :: RenamedExpr -> TcM TypecheckedExpr

  Much less clutter in the code, and more efficient too.
  (Originally suggested by Mark Shields.)


		-----------------
		Remove "SysNames"
		-----------------

Because the renamer and the type checker were entirely separate,
we had to carry some rather tiresome implicit binders (or "SysNames")
along inside some of the HsDecl data structures.  They were both
tiresome and fragile.

Now that the typechecker and renamer are more intimately coupled,
we can eliminate SysNames (well, mostly... default methods still
carry something similar).

		-------------
		Clean up HsPat
		-------------

One big clean up is this: instead of having two HsPat types (InPat and
OutPat), they are now combined into one.  This is more consistent with
the way that HsExpr etc is handled; there are some 'Out' constructors
for the type checker output.

So:
	HsPat.InPat	--> HsPat.Pat
	HsPat.OutPat	--> HsPat.Pat
	No 'pat' type parameter in HsExpr, HsBinds, etc

	Constructor patterns are nicer now: they use
		HsPat.HsConDetails
	for the three cases of constructor patterns:
		prefix, infix, and record-bindings

	The *same* data type HsConDetails is used in the type
	declaration of the data type (HsDecls.TyData)

Lots of associated clean-up operations here and there.  Less code.
Everything is wonderful.

FastString cleanup, stage 1.

The FastString type is no longer a mixture of hashed strings and
literal strings, it contains hashed strings only with O(1) comparison
(except for UnicodeStr, but that will also go away in due course).  To
create a literal instance of FastString, use FSLIT("..").

By far the most common use of the old literal version of FastString
was in the pattern

	  ptext SLIT("...")

this combination still works, although it doesn't go via FastString
any more.  The next stage will be to remove the need to use this
special combination at all, using a RULE.

To convert a FastString into an SDoc, now use 'ftext' instead of
'ptext'.

I've also removed all the FAST_STRING related macros from HsVersions.h
except for SLIT and FSLIT, just use the relevant functions from
FastString instead.

------------------------------------
	Change the treatment of the stupid
	   context on data constructors
	-----------------------------------

Data types can have a context:

	data (Eq a, Ord b) => T a b = T1 a b | T2 a

and that makes the constructors have a context too
(notice that T2's context is "thinned"):

	T1 :: (Eq a, Ord b) => a -> b -> T a b
	T2 :: (Eq a) => a -> T a b

Furthermore, this context pops up when pattern matching
(though GHC hasn't implemented this, but it is in H98, and
I've fixed GHC so that it now does):

	f (T2 x) = x
gets inferred type
	f :: Eq a => T a b -> a

I say the context is "stupid" because the dictionaries passed
are immediately discarded -- they do nothing and have no benefit.
It's a flaw in the language.

Up to now I have put this stupid context into the type of
the "wrapper" constructors functions, T1 and T2, but that turned
out to be jolly inconvenient for generics, and record update, and
other functions that build values of type T (because they don't
have suitable dictionaries available).

So now I've taken the stupid context out.  I simply deal with
it separately in the type checker on occurrences of a constructor,
either in an expression or in a pattern.

To this end

* Lots of changes in DataCon, MkId

* New function Inst.tcInstDataCon to instantiate a data constructor



I also took the opportunity to

* Rename
	dataConId --> dataConWorkId
  for consistency.

* Tidied up MkId.rebuildConArgs quite a bit, and renamed it
	mkReboxingAlt

* Add function DataCon.dataConExistentialTyVars, with the obvious meaning

imports

Tidier printing routines for Rules

Misc cleanup: remove the iface pretty-printing style, and clean up
bits of StringBuffer that aren't required any more.

------------------------
	Change
		GlobalName --> ExternalName
		LocalName  ->  InternalName
	------------------------

For a long time there's been terminological confusion between

	GlobalName vs LocalName	 (property of a Name)
	GlobalId vs LocalId	 (property of an Id)

I've now changed the terminology for Name to be

	ExternalName vs InternalName

I've also added quite a bit of documentation in the Commentary.

Binary Interface Files - stage 1
--------------------------------

This commit changes the default interface file format from text to
binary, in order to improve compilation performace.

To view an interface file, use 'ghc --show-iface Foo.hi'.

utils/Binary.hs is the basic Binary I/O library, based on the nhc98
binary I/O library but much stripped-down and working in terms of
bytes rather than bits, and with some special features for GHC: it
remembers which Module is being emitted to avoid dumping too many
qualified names, and it keeps track of a "dictionary" of FastStrings
so that we don't dump the same FastString more than once into the
binary file.  I'll make a generic version of this for the libraries at
some point.

main/BinIface.hs contains most of the Binary instances.  Some
instances are in the same module as the data type (RdrName, Name,
OccName in particular).  Most instances were generated using a
modified version of DrIFT, which I'll commit later.  However, editing
them by hand isn't hard (certainly easier than modifying
ParseIface.y).

The first thing in a binary interface is the interface version, so
nice error messages will be generated if the binary format changes and
you still have old interfaces lying around.  The version also now
includes the "way" as an extra sanity check.

Other changes
-------------

I don't like the way FastStrings contain both hashed strings (with
O(1) comparison) and literal C strings (with O(n) comparison).  So as
a first step to separating these I made serveral "literal" type
strings into hashed strings.  SLIT() still generates a literal, and
now FSLIT() generates a hashed string.  With DEBUG on, you'll get a
warning if you try to compare any SLIT()s with anything, and the
compiler will fall over if you try to dump any literal C strings into
an interface file (usually indicating a use of SLIT() which should be
FSLIT()).

mkSysLocal no longer re-encodes its FastString argument each time it
is called.

I also fixed the -pgm options so that the argument can now optionally
be separted from the option.

Bugfix: PrelNames declared Names for several comparison primops, eg.
eqCharName, eqIntName etc. but these had different uniques from the
real primop names.  I've moved these to PrimOps and defined them using
mkPrimOpIdName instead, and deleted some for which we don't have real
primops (Manuel: please check that things still work for you after
this change).

comments

----------------------------------
	Do the Right Thing for TyCons where we
	can't see all their constructors.
	----------------------------------

Inside a TyCon, three things can happen

1. GHC knows all the constructors, and has them to hand.
   (Nowadays, there may be zero constructors.)

2. GHC knows all the constructors, but has declined to slurp
   them all in, to avoid sucking in more declarations than
   necessary.  All we remember is the number of constructors,
   so we can get the return convention right.

3. GHC doesn't know anything. This happens *only* for decls
   coming from .hi-boot files, where the programmer declines to
   supply a representation.

Until now, these three cases have been conflated together.  Matters
are worse now that a TyCon really can have zero constructors.  In
fact, by confusing (3) with (1) we can actually generate bogus code.

With this commit, the dataCons field of a TyCon is of type:

data DataConDetails datacon
  = DataCons [datacon]	-- Its data constructors, with fully polymorphic types
			-- A type can have zero constructors

  | Unknown		-- We're importing this data type from an hi-boot file
			-- and we don't know what its constructors are

  | HasCons Int		-- In a quest for compilation speed we have imported
			-- only the number of constructors (to get return
			-- conventions right) but not the constructors themselves

This says exactly what is going on.  There are lots of consequential small
changes.

---------
		Main.main
		---------

A bunch of related fixes concerning 'main'

* Arrange that 'main' doesn't need to be defined in module Main;
  it can be imported.

* The typechecker now injects a binding
	Main.$main = PrelTopHandler.runMain main

  So the runtime system now calls Main.$main, not PrelMain.main.
  With z-encoding, this look like
		Main_zdmain_closure

* The function
  	PrelTopHandler.runMain :: IO a -> IO ()
  wraps the programmer's 'main' in an exception-cacthing wrapper.

* PrelMain.hs and Main.hi-boot are both removed from lib/std, along
  with multiple lines of special case handling in lib/std/Makefile.
  This is a worthwhile cleanup.

* Since we now pick up whatever 'main' is in scope, the ranamer gets
  in on the act (RnRnv.checkMain).  There is a little more info to
  get from the renamer to the typechecker, so I've defined a new type
  Rename.RnResult (c.f. TcModule.TcResult)

* With GHCi, it's now a warning, not an error, to omit the binding
  of main (RnEnv.checkMain)

* It would be easy to add a flag "-main-is foo"; the place to use
  that information is in RnEnv.checkMain.


-------

On the way I made a new type,
	type HscTypes.FixityEnv = NameEnv Fixity
and used it in various places I'd tripped over

Simplify the package story inside the compiler.  The new story is
this:

  The Finder no longer determines a module's package based on its
  filesystem location.  The filesystem location indicates only whether
  a given module is in the current package or not (i.e. found along
  the -i path ==> current package, found along the package path ==>
  other package).

  Hence a Module no longer contains a package name.  Instead it just
  contains PackageInfo, which is either ThisPackage or AnotherPackage.
  The compiler uses this information for generating cross-DLL calls
  and omitting certain version information from .hi files.

  The interface still contains the package name.  This isn't used for
  anything right now, but in the future (when we have hierarchical
  libraries) we might use it to automatically determine which packages
  a binary should be linked against.  When building a package, you
  should still use -package-name, but it won't be fatal if you don't.

The warning/error about package name mismatches has gone away.

Add support for Hugs's :browse (or :b) command.  There are two forms:

	- :b M   (interpreted modules only) shows everything
          defined in M - the types of top-level functions,
	  and definitions of classes and datatypes.

	- :b *M  shows everything exported from module M.
	  Available for both compiled and interpreted modules.

The user interface is subject to change, but for now it is consistent
with the new semantics of the :module command.

The implementation is a little tricky, since for a package module we
have to be sure to slurp in all the required declarations first.

Make some record selections strict to reduce space leaks.

follow-on from prev. commit; more tidyups

NB:	(Just a) `mplus` (Just b) = a

We want catMaybes, not mplus!

- ifaceTyThing: avoid using (++) when constructing the IdInfo for AnId - Maybe
                is preferable for this.
- misc typesig tidy-ups to make easier to get into the workings of the
  functions herein.

----------------------------------------------
	The CoreTidy/CorePrep/CoreToStg saga continues
	[actually, this commit mostly completes the job]
	----------------------------------------------

			DO NOT MERGE!

* CorePrep injects implicit bindings, not the type checker,
  nor CgConTbls.   (This way, all the code generators see
  them, so no need to fiddle with the byte code generator.)

  As a result, all bindings in the module are for LocalIds,
  at least until CoreTidy.   This is a Big Win.

  Hence remove nasty isImplicitId test in update_bndr in
  SimplCore and DmdAnal

* hasNoBinding is no longer true of a dataConId (worker).
  There's an implicit curried binding for it.

* Remove yukky test in exprIsTrivial that did not regard
  a hasNoBinding Id as trivial; similarly in SimplUtils.tryEtaReduce

* In CoreTidy, get the names to avoid from the type env.
  That way it includes implicit bindings too.

* CoreTidy set the Arity of a top-level Id permanently;
  it's up to the rest of the compiler to respect it.
  Notably, CorePrep uses etaExpand to make the manifest arity
  match the claimed arity.

* As a result, nuke CgArity, so that CgInfo now contains only
  CafInfo.  The CafInfo is knot-tied as before.


Other things

* In Simplify.simplLazyBind, be a bit keener to float bindings
  out if it's a top-level binding.

Rename ifaceTyCls to ifaceTyThing (more consistent)

Heal the HEAD

------------------
		Simon's big commit
		------------------

This commit, which I don't think I can sensibly do piecemeal, consists
of the things I've been doing recently, mainly directed at making
Manuel, George, and Marcin happier with RULES.


Reogranise the simplifier
~~~~~~~~~~~~~~~~~~~~~~~~~
1. The simplifier's environment is now an explicit parameter.  This
makes it a bit easier to figure out where it is going.

2. Constructor arguments can now be arbitrary expressions, except
when the application is the RHS of a let(rec).  This makes it much
easier to match rules like

	RULES
	    "foo"  f (h x, g y) = f' x y

In the simplifier, it's Simplify.mkAtomicArgs that ANF-ises a
constructor application where necessary.  In the occurrence analyser,
there's a new piece of context info (OccEncl) to say whether a
constructor app is in a place where it should be in ANF.  (Unless
it knows this it'll give occurrence info which will inline the
argument back into the constructor app.)

3. I'm experimenting with doing the "float-past big lambda" transformation
in the full laziness pass, rather than mixed in with the simplifier (was
tryRhsTyLam).

4.  Arrange that
	case (coerce (S,T) (x,y)) of ...
will simplify.  Previous it didn't.
A local change to CoreUtils.exprIsConApp_maybe.

5. Do a better job in CoreUtils.exprEtaExpandArity when there's an
error function in one branch.


Phase numbers, RULES, and INLINE pragmas
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1.  Phase numbers decrease from N towards zero (instead of increasing).
This makes it easier to add new earlier phases, which is what users want
to do.

2.  RULES get their own phase number, N, and are disabled in phases before N.

e.g. 	{-# RULES "foo" [2] forall x y.  f (x,y) = f' x y #-}

Note the [2], which says "only active in phase 2 and later".

3.  INLINE and NOINLINE pragmas have a phase number to.  This is now treated
in just the same way as the phase number on RULE; that is, the Id is not inlined
in phases earlier than N.  In phase N and later the Id *may* be inlined, and
here is where INLINE and NOINLINE differ: INLNE makes the RHS look small, so
as soon as it *may* be inlined it probably *will* be inlined.

The syntax of the phase number on an INLINE/NOINLINE pragma has changed to be
like the RULES case (i.e. in square brackets).  This should also make sure
you examine all such phase numbers; many will need to change now the numbering
is reversed.

Inlining Ids is no longer affected at all by whether the Id appears on the
LHS of a rule.  Now it's up to the programmer to put a suitable INLINE/NOINLINE
pragma to stop it being inlined too early.


Implementation notes:

*  A new data type, BasicTypes.Activation says when a rule or inline pragma
is active.   Functions isAlwaysActive, isNeverActive, isActive, do the
obvious thing (all in BasicTypes).

* Slight change in the SimplifierSwitch data type, which led to a lot of
simplifier-specific code moving from CmdLineOpts to SimplMonad; a Good Thing.

* The InlinePragma in the IdInfo of an Id is now simply an Activation saying
when the Id can be inlined.  (It used to be a rather bizarre pair of a
Bool and a (Maybe Phase), so this is much much easier to understand.)

* The simplifier has a "mode" environment switch, replacing the old
black list.  Unfortunately the data type decl has to be in
CmdLineOpts, because it's an argument to the CoreDoSimplify switch

    data SimplifierMode = SimplGently | SimplPhase Int

Here "gently" means "no rules, no inlining".   All the crucial
inlining decisions are now collected together in SimplMonad
(preInlineUnconditionally, postInlineUnconditionally, activeInline,
activeRule).


Specialisation
~~~~~~~~~~~~~~
1.  Only dictionary *functions* are made INLINE, not dictionaries that
have no parameters.  (This inline-dictionary-function thing is Marcin's
idea and I'm still not sure whether it's a good idea.  But it's definitely
a Bad Idea when there are no arguments.)

2.  Be prepared to specialise an INLINE function: an easy fix in
Specialise.lhs

But there is still a problem, which is that the INLINE wins
at the call site, so we don't use the specialised version anyway.
I'm still unsure whether it makes sense to SPECIALISE something
you want to INLINE.





Random smaller things
~~~~~~~~~~~~~~~~~~~~~~

* builtinRules (there was only one, but may be more) in PrelRules are now
  incorporated.   They were being ignored before...

* OrdList.foldOL -->  OrdList.foldrOL, OrdList.foldlOL

* Some tidying up of the tidyOpenTyVar, tidyTyVar functions.  I've
  forgotten exactly what!

--------------------------
	Add a rule-check pass
	(special request by Manuel)
	--------------------------

	DO NOT merge with stable

The flag

	-frule-check foo

will report all sites at which RULES whose name starts with "foo.."
might apply, but in fact the arguments don't match so the rule
doesn't apply.

The pass is run right after all the core-to-core passes.  (Next thing
to do: make the core-to-core script external, so you can fiddle with
it.  Meanwhile, the core-to-core script is in
	DriverState.builCoreToDo
so you can move the CoreDoRuleCheck line around if you want.

The format of the report is experimental: Manuel, feel free to fiddle
with it.

Most of the code is in specialise/Rules.lhs


Incidental changes
~~~~~~~~~~~~~~~~~~
Change BuiltinRule so that the rule name is accessible
without actually successfully applying the rule.  This
change affects quite a few files in a trivial way.

Do something vaguely useful in ifaceTyCls when presented with a
primitive type constructor.  We pretend it's an abstract data type for
now.

Implement the :info command for GHCi.

---------------------------------
	Switch to the new demand analyser
	---------------------------------

This commit makes the new demand analyser the main beast,
with the old strictness analyser as a backup.  When
DEBUG is on, the old strictness analyser is run too, and the
results compared.

WARNING: this isn't thorougly tested yet, so expect glitches.
Delay updating for a few days if the HEAD is mission critical
for you.

But do try it out.  I'm away for 2.5 weeks from Thursday, so
it would be good to shake out any glaring bugs before then.

Add support for Hugs's :info command. Doesn't work yet, but shouldn't
interfere with anything else. Some of the files touched are just to correct
out-of-date comments.

Highlights are:

hscThing: like hscStmt, but just gets info about a single identifier
cmInfoThing: exposes hscThing's functionality to the outside world