Cleanup pass on functional dependencies.  Most noticeably, make it so that
signatures involving classes with functional dependencies work.  Also,
Fundeps are now properly handled by the simplifier, resolving problems
where the fundeps were sometimes being discarded too early, and sometimes
hanging around too long.  Took out the early ambiguity testing in the
renamer, because that's too early (you don't know the fundeps yet).  Now,
the ambiguity test happens in the typechecker.
Functional Dependencies should now be up to snuff with Mark's paper,
however, the derived instances and superclass extensions found in hugs
are still not in there.
It would be nice if this were merged into 4.07.  I have diffs against
the 4.07 tree in case it's too thorny working around Simon's big commit.