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Based on a patch from David Terei.

Some parts are a little ugly (e.g. defining things that only ASSERTs
use only when DEBUG is defined), so we might want to tweak things a
little.

I've also turned off -Werror for didn't-inline warnings, as we now
get a few such warnings.

This type was mainly there to support silent superclass
parameters for dfuns, and they have gone away.  So this
patch is another minor simplification.

(Interface format change; you need to make clean.)

Before this change the constant expression

   _ccI::I64 = (16 >> 7) + 1;

wouldn't be propagated, as it wouldn't be completely folded.  This
meant that this expression wouldn't be unrolled

    thawArray# arr# 0# 16# s#

The new code generator already does this correctly.

This is a little cleaner, and removes a barrier to cross-compiling
to/from Windows. There are a few more tools that still need to be
handled in the same way.

Hurrah.  At last we can write

   class (F a ~ b) => C a b where { ... }

This fruit of the fact that equalities are now values,
and all evidence is handled uniformly.

The main tricky point is that when translating to Core
an evidence variable 'v' is represented either as
  either   Var v
  or       Coercion (CoVar v)
depending on whether or not v is an equality.  This leads
to a few annoying calls to 'varToCoreExpr'.

We introduced silent superclass parameters as a way to avoid
superclass loops, but we now solve that problem a different
way ("derived" superclass constraints carry no evidence). So
they aren't needed any more.

Apart from being a needless complication, they broke DoCon.
Admittedly in a very obscure way, but still the result is
hard to explain. To see the details see Trac #5051, with
test case typecheck/should_compile/T5051.  (The test is
nice and small!)

While we previously checked the safety of safe imported modules we
didn't do this check transitively. This can be a problem when we depend
on a trustworthy module in a package that is no longer trusted, so we
should fail compilation. We already stored in an interface file the
transitive list of packages a module depends on. Now we extend that list
to include a flag saying if we depend on that package being trusted as
well.

Comment was removed as I thought it was incorrect now with code
changes but Simon M thinks my interpretation of the comment
was incorrect. He isn't sure what the comment is refereeing to
either though! So have restored comment and created trac #5249
to track fixing this at some point.