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.