Typechecking [TcModule, TcBinds, TcHsSyn, TcInstDcls, TcSimplify]
~~~~~~~~~~~~
* Fix a bug in TcSimplify that broke functional dependencies.
  Interleaving unification and context reduction is trickier 
  than I thought.  Comments in the code amplify.  

* Fix a functional-dependency bug, that meant that this pgm:
	class C a b | a -> b where f :: a -> b
	
	g :: (C a b, Eq b) => a -> Bool
	g x = f x == f x
  gave an ambiguity error report.  I'm afraid I've forgotten
  what the problem was.


* Correct the implementation of the monomorphism restriction,
  in TcBinds.generalise.  This fixes Marcin's bug report:
	test1 :: Eq a => a -> b -> b
	test1 x y = y

	test2 = test1 (3::Int)
  Previously we were erroneously inferring test2 :: () -> ()

* Make the "unf_env" that is looped round in TcModule go round
  in a big loop, not just round tcImports.  This matters when
  we have mutually recursive modules, so that the Ids bound in
  the source code may appear in the imports.  Sigh.  But no big
  deal.

  It does mean that you have to be careful not to call isLocalId,
  isDataConId etc, because they consult the IdInfo of an Id, which 
  in turn may be determined by the loop-tied unf_env.