Fix rank-validity testing

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.