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With this change it should be possible to perform something similar to
'load' by traversing the module graph in dependency order and calling
'{parse,typecheck,load}Module' on each.  Of course, if you want smart
recompilation checking you should still use 'load'.

parameter to 'InteractiveStatus' to a 'Maybe'.

This patch entails a major restructuring of HscMain and a small bugfix
to MkIface (which required the restructuring in HscMain).

In MkIface:

  - mkIface* no longer outputs orphan warnings directly and also no
    longer quits GHC when -Werror is set.  Instead, errors are
    reported using the common IO (Messages, Maybe result) scheme.

In HscMain:

  - Get rid of the 'Comp' monad.  This monad was mostly GhcMonad + two
    reader arguments, a ModSummary for the currently compiled module
    and a possible old interface.  The latter actually lead to a small
    space-leak since only its hash was needed (to check whether the
    newly-generated interface file was the same as the original one).

    Functions originally of type 'Comp' now only take the arguments
    that they actually need.  This leads to slighly longer argument
    lists in some places, however, it is now much easier to see what
    is actually going on.

  - Get rid of 'myParseModule'.  Rename 'parseFile' to 'hscParse'.

  - Join 'deSugarModule' and 'hscDesugar' (keeping the latter).

  - Rename 'typecheck{Rename}Module{'}' to 'hscTypecheck{Rename}'.
    One variant keeps the renamed syntax, the other doesn't.

  - Parameterise 'HscStatus', so that 'InteractiveStatus' is just a
    different parameterisation of 'HscStatus'.

  - 'hscCompile{OneShot,Batch,Nothing,Interactive}' are now
    implemented using a (local) typeclass called 'HsCompiler'.  The
    idea is to make the common structure more obvious.  Using this
    typeclass we now have two functions 'genericHscCompile' (original
    'hscCompiler') and 'genericHscRecompile' (original 'genComp')
    describing the default pipeline.  The methods of the typeclass
    describe a sort of "hook" interface (in OO-terms this would be
    called the "template method" pattern).

    One problem with this approach is that we parameterise over the
    /result/ type which, in fact, is not actually different for
    "nothing" and "batch" mode.  To avoid functional dependencies or
    associated types, we use type tags to make them artificially
    different and parameterise the type class over the result type.
    A perhaps better approach might be to use records instead.
    
  - Drop some redundant 'HscEnv' arguments.  These were likely
    different from what 'getSession' would return because during
    compilation we temporarily set the module's DynFlags as well as a
    few other fields.  We now use the 'withTempSession' combinator to
    temporarily change the 'HscEnv' and automatically restore the
    original session after the enclosed action has returned (even in
    case of exceptions).

  - Rename 'hscCompile' to 'hscGenHardCode' (since that is what it
    does).

Calls in 'GHC' and 'DriverPipeline' accordingly needed small
adaptions.

It now uses the standard warning log and error reporting mechanism.

Parse errors during dependency analysis or options parsing really
shouldn't kill GHC; this is particularly annoying for GHC API clients.

The sense of the #ifdef was wrong

Do not filter the rts from linked libraries in linkDynLib as Windows does not allow unresolved symbols

When linking in --make we check the modification time of the
executable against the modification time of the object files, and only
re-link if any object file is newer.  However, we should also check
the modification times of packages, since the recompilation checker
also tracks dependencies in packages.  

In a GHC build this means that if you recompile stage2 and don't
manage to change any fingerpints, we won't recompile Main but we'll
still re-link it.

Now that we don't include any header files in .hc apart from our own,
the cc-options from packages are at best superfluous, so don't pass
them.

We still pass them to .c compilations, although I've just made changes
to Cabal so that cc-options from a .cabal file are not copied into the
InstalledPackageInfo.  Most uses of cc-options in Cabal are clearly
intended to be local to the package, but they were being propagated
everywhere, almost certainly unintentionally.

The way is left open for Cabal to allow packages to specify cc-options
that get propagated in the future, if we find a use case for this.

TopHandler now uses the new extensible exceptions module, so we
need to interact with it using the new types.

We want to be able to pick the RTS flavour (e.g. -threaded) when we
link the final program.

These are needed for GLOBAL_VAR's to work properly

so we no longer need to use defaultDynFlags there

Several changes in this patch, partially bug fixes, partially new code:
o bug fixes in ZipDataflow
   - added some checks to verify that facts converge
   - removed some erroneous checks of convergence on entry nodes
   - added some missing applications of transfer functions
o changed dataflow clients to use ZipDataflow, making ZipDataflow0 obsolete
o eliminated DFA monad (no need for separate analysis and rewriting monads with ZipDataflow)
o started stack layout changes
   - no longer generating CopyIn and CopyOut nodes (not yet fully expunged though)
   - still not using proper calling conventions
o simple new optimizations:
   - common block elimination
      -- have not yet tried to move the Adams opt out of CmmProcPointZ
   - block concatenation
o piped optimization fuel up to the HscEnv
   - can be limited by a command-line flag
   - not tested, and probably not yet properly used by clients
o added unique supply to FuelMonad, also lifted unique supply to DFMonad

Modules that need it import it themselves instead.

These fix these failures:
   break008(ghci)
   break009(ghci)
   break026(ghci)
   ghci.prog009(ghci)
   ghci025(ghci)
   print007(ghci)
   prog001(ghci)
   prog002(ghci)
   prog003(ghci)
at least some of which have this symptom:
    Exception: expectJust prune

Added API support for compiling Haskell to simplified Core, and for
compiling Core to machine code. The latter, especially, should be
considered experimental and has only been given cursory testing. Also
fixed warnings in DriverPipeline. Merry Christmas.

Using -stubdir together with hierarchical modules, -fvia-C, and --make
is essentially broken in 6.6.x.  Recently discovered by Cabal's use of
-stubdir.

Test cases: driver027/driver028 (I've updated them to use -fvia-C, in
order to test for this bug).