GHC does not now do "hoisting" as it used to.  Instead, it allows
foralls to the right of fuction arrows, as well as to the left.

But the type-validity tester hadn't caught up.  This commit fixes 
it. The test is tc203.

Incidentally, GHC still doesn't let you write
	forall a. Eq a => forall b. b -> b
because we get a zillion reduce/reduce errors if we allow that.  I'm
sure it's fixable.  But meanwhile you have to use an auxiliary type
synonym, which is a bit stupid.