Update for QuantifiedConstraints

[treeowl]

QuantifiedConstraints makes some parts of this package rather less exciting (in particular, I believe everything currently in Data.Constraint.Forall can be made Safe). On the other hand, it introduces its own gaps. For example, we can easily write

weaken :: (c => d) => c :- d
weaken = Sub Dict

But we can't write the equally reasonable

strengthen :: ((c => d) => r) -> c :- d -> r

Doing so requires some sort of coercion that's not immediately obvious; it'd be nice if constraints could help with that.