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Introduce new flags to allow any package database stack to be set up.
The `-no-user-package-conf` and `-no-global-package-conf` flags remove
the corresponding package db from the initial stack, while
`-user-package-conf` and `-global-package-conf` push it back on top of
the stack.

Patch by Sam Anklesaria <amsay@amsay.net>

No reason user should turn them off, just want for ghc developer
debugging uses.

This extension is implied by:

  * TypeOperators: so that we can import/export things like (+)
  * TypeFamilies: because associated type synonyms use "type T"
                  to name the associated type in a subordinate list.

* Enables vectorisation avoidance

(This commit only adds the flag, not the actual vectorisation avoidance transformation.)

GHCi now maintains two DynFlags: one that applies to whole modules
loaded with :load, and one that applies to things typed at the prompt
(expressions, statements, declarations, commands).

  The :set command modifies both DynFlags.  This is for backwards
  compatibility: users won't notice any difference.

  The :seti command applies only to the interactive DynFlags.

Additionally, I made a few changes to ":set" (with no arguments):

  * Now it only prints out options that differ from the defaults,
    rather than the whole list.

  * There is a new variant, ":set -a" to print out all options (the
    old behaviour).

  * It also prints out language options.

e.g.

Prelude> :set
options currently set: none.
base language is: Haskell2010
with the following modifiers:
  -XNoDatatypeContexts
  -XNondecreasingIndentation
GHCi-specific dynamic flag settings:
other dynamic, non-language, flag settings:
  -fimplicit-import-qualified
warning settings:

":seti" (with no arguments) does the same as ":set", but for the
interactive options.  It also has the "-a" option.

The interactive DynFlags are kept in the InteractiveContext, and
copied into the HscEnv at the appropriate points (all in HscMain).

There are some new GHC API operations:

-- | Set the 'DynFlags' used to evaluate interactive expressions.
setInteractiveDynFlags :: GhcMonad m => DynFlags -> m ()

-- | Get the 'DynFlags' used to evaluate interactive expressions.
getInteractiveDynFlags :: GhcMonad m => m DynFlags

-- | Sets the program 'DynFlags'.
setProgramDynFlags :: GhcMonad m => DynFlags -> m [PackageId]

-- | Returns the program 'DynFlags'.
getProgramDynFlags :: GhcMonad m => m DynFlags

Note I have not completed the whole of the plan outlined in #3217 yet:
when in the context of a loaded module we don't take the interactive
DynFlags from that module.  That needs some more refactoring and
thinking about, because we'll need to save and restore the original
interactive DynFlags.

This solves the immediate problem that people are having with the new
flag checking in 7.4.1, because now it is possible to set language
options in ~/.ghci that do not affect loaded modules and thereby cause
recompilation.

stdout/stderr might be closed, so we can't just hFlush them.
So we instead allow configuration in the same way that log_action
is configurable.

Add a missing 'be' to Haddock comment of `HscTarget`.

This patch defines a flag -fno-warn-pointless-pragmas, and uses it to
disable some warnings in the containers package.

Along the way, also made a ContainsDynFlags class, and added a
HasDynFlags instance for IOEnv (and thus TcRnIf and DsM).

is used for optimisation. (enabled by default)

This patch implements the idea of deferring (most) type errors to
runtime, instead emitting only a warning at compile time.  The
basic idea is very simple:

 * The on-the-fly unifier in TcUnify never fails; instead if it
   gets stuck it emits a constraint.

 * The constraint solver tries to solve the constraints (and is
   entirely unchanged, hooray).

 * The remaining, unsolved constraints (if any) are passed to
   TcErrors.reportUnsolved.  With -fdefer-type-errors, instead of
   emitting an error message, TcErrors emits a warning, AND emits
   a binding for the constraint witness, binding it
   to (error "the error message"), via the new form of evidence
   TcEvidence.EvDelayedError.  So, when the program is run,
   when (and only when) that witness is needed, the program will
   crash with the exact same error message that would have been
   given at compile time.

Simple really.  But, needless to say, the exercise forced me
into some major refactoring.

 * TcErrors is almost entirely rewritten

 * EvVarX and WantedEvVar have gone away entirely

 * ErrUtils is changed a bit:
     * New Severity field in ErrMsg
     * Renamed the type Message to MsgDoc (this change
       touches a lot of files trivially)

 * One minor change is that in the constraint solver we try
   NOT to combine insoluble constraints, like Int~Bool, else
   all such type errors get combined together and result in
   only one error message!

 * I moved some definitions from TcSMonad to TcRnTypes,
   where they seem to belong more

We no longer have many separate, clashing getDynFlags functions

I've given each GhcMonad its own HasDynFlags instance, rather than
using UndecidableInstances to make a GhcMonad m => HasDynFlags m
instance.

This new feature-ette, enable with -XInstanceSigs, lets
you give a type signature in an instance declaration:

   instance Eq Int where
     (==) :: Int -> Int -> Bool
     (==) = ...blah...

Scoped type variables work too.

  - Attach a SrcSpan to every CostCentre.  This had the side effect
    that CostCentres that used to be merged because they had the same
    name are now considered distinct; so I had to add a Unique to
    CostCentre to give them distinct object-code symbols.

  - New flag: -fprof-auto-calls.  This flag adds an automatic SCC to
    every call site (application, to be precise).  This is typically
    more useful for call stacks than annotating whole functions.

Various tidy-ups at the same time: removed unused NoCostCentre
constructor, and refactored a bit in Coverage.lhs.

The call stack we get from traceStack now looks like this:

Stack trace:
  Main.CAF (<entire-module>)
  Main.main.xs (callstack002.hs:18:12-24)
  Main.map (callstack002.hs:13:12-16)
  Main.map.go (callstack002.hs:15:21-34)
  Main.map.go (callstack002.hs:15:21-23)
  Main.f (callstack002.hs:10:7-43)

In GHC, this provides an easy way to call a C function via a C wrapper.
This is important when the function is really defined by CPP.

Requires the new CApiFFI extension.

Not documented yet, as it's still an experimental feature at this stage.

This improves compile times slightly.

LLVM doesn't support the OS X system assembler anymore so we must use
their assembler through clang. Also improved error messages when various
LLVM tools can't be run.

-ddump-* files are put in the given directory.

-outputdir also sets the dumpdir.

I did a bit of refactoring (of course) at the same time.
See the discussion in Trac #5587.  Most of the real change
is in CoreArity.

This big patch implements a kind-polymorphic core for GHC. The current
implementation focuses on making sure that all kind-monomorphic programs still
work in the new core; it is not yet guaranteed that kind-polymorphic programs
(using the new -XPolyKinds flag) will work.

For more information, see http://haskell.org/haskellwiki/GHC/Kinds

Now included:
    - all language flags
    - all filename-related flags (-i, -osuf, -hidir etc.)
    - all CPP-related flags (-I, -D, -U)

This means ghci now correctly picks up the package flags
for tab completion.