Now that the in-tree gmp has been upgraded, it should no longer be
necessary, according to #5293.

See Trac #5348

An claimed short-cut optimisation was actually an error.
The optimisation was this: when looking up (C a b), where
'a' and 'b' are type variables, we were returning [] immediately
if the instance environment had no instances of form (C a b).
Why? Because the thing being looked up definitely won't match
(C Int Bool), say.

BUT it will *unify* with (C Int Bool) and we care very much
about things it might unify with.  If we neglect them we may
silently allow incoherent instance selection, and that is
exactly what happened in #5095.

The fix is easy: remove the "optimisation".

The type signatures are much clearer, but need ScopedTypeVariables.
Happily that is now available in our bootstrap compilers.

There is a long discussion in the ticket.

rather than using isPredTy!  In Core land, a PredTy
and its representation type are synonymous.

These turn out to be a useful special case of splitTyConApp_maybe.

A refactoring only; no change in behaviour

I also tidied up the interfaces for LoadIface to be a bit simpler

of the RTS addDLL() API on Windows.  When searching for DLLs we should
include the .dll extension, but addDLL() takes a filename without the
extension.

linking scenarios.  We weren't searching for .a archives to satisfy
-lfoo options on the GHCi command line, for example.

I've tidied up the code in this module so that dealing with -l options
on the command line is consistent with the handling of extra-libraries
for packages.

While I was here I moved some stuff out of Linker.hs that didn't seem
to belong here: dataConInfoPtrToName (now in new module DebuggerUtils)
and lessUnsafeCoerce (now in DynamicLoading, next to its only use)

This fixes Trac #5147, which was going wrong because
the class ABI fingerprint wasn't changing when we added
or removed a Show instance to the associated type.

It's needed in ghc/InteractiveUI, although not in the compiler itself

See Note [Internal used_names].  Fixes Trac #5362.

Currently export list in .hi files are partitioned by module
  export M T(C1,C2)
         N f,g
In each list we only have OccNames, all assumed to come from
the parent module M or N resp.

This patch changes the representatation so that export lists
have full Names:
  export M.T(M.C1,M.C2), N.f, N.g

Numerous advatages
  * AvailInfo no longer needs to be parameterised; it always
    contains Names

  * Fixes Trac #5306.  This was the main provocation

  * Less to-and-fro conversion when reading interface files

It's all generally simpler.  Interface files should not get bigger,
becuase they have a nice compact representation for Names.

Instead of two fields
   ic_toplev_scope :: [Module]
   ic_imports      :: [ImportDecl RdrName]

we now just have one
   ic_imports :: [InteractiveImport]
with the auxiliary data type
   data InteractiveImport
    = IIDecl (ImportDecl RdrName)  -- Bring the exports of a particular module
    	   	       		   -- (filtered by an import decl) into scope

    | IIModule Module	-- Bring into scope the entire top-level envt of
    	     		-- of this module, including the things imported
			-- into it.

This makes lots of code less confusing.  No change in behaviour.
It's preparatory to fixing Trac #5147.

While I was at I also
  * Cleaned up the handling of the "implicit" Prelude import
    by adding a ideclImplicit field to ImportDecl.  This
    significantly reduces plumbing in the handling of
    the implicit Prelude import

  * Used record notation consistently for ImportDecl

Fixes Trac #5359.