This patch improves the subsumption check (in TcUnify.tc_sub) so that
it does pre-subsumption first.  The key code is in the case with
guard (isSigmaTy actual_ty); note the new call to preSubType.

Shorn of details, the question is this.  Should this hold?

	forall a. a->a   <=   Int -> (forall b. Int)

Really, it should; just instantiate 'a' to Int.  This is just what
the pre-subsumption phase (which used in function applications),
will do.

I did a bit of refactoring to achieve this.

Fixes Trac #821.  Test tc205 tests.