----------------------------------
	Improve higher-rank type inference
	----------------------------------

Yanling Wang pointed out that if we have

	f = \ (x :: forall a. a->a). x

it would be reasonable to expect that type inference would get the "right"
rank-2 type for f.  She also found that the plausible definition

	f :: (forall a. a->a) = \x -> x

acutally failed to type check.

This commit fixes up TcBinds.tcMonoBinds so that it does a better job.
The main idea is that there are three cases to consider in a function binding:

  a) 'f' has a separate type signature
	In this case, we know f's type everywhere

  b) The binding is recursive, and there is no type sig
	In this case we must give f a monotype in its RHS

  c) The binding is non-recursive, and there is no type sig
	Then we do not need to add 'f' to the envt, and can
	simply infer a type for the RHS, which may be higher
	ranked.