Improve the rule-matcher
Previously it was rejecting the match Template: forall s t. map s t Actual: map Int t which should obviously be fine. It turns out that this kind of match comes up when specialising. By freshening that t we could avoid the difficulty, but morally the (forall t) binds t and the rule should be alpha-equivalent regardless of the forall'd variables. This patch makes it so, and incidentally makes matching a little more efficient. See Note [Eta expansion] in VarEnv.