Move Yoke over to GATs

[Manishearth]

<T as Yokeable<'a>>::Output is just a way of saying Self<'a> in a GAT-less world.

Now that GATs are rather stable and well inside our MSRV, we should use them.

[Manishearth]

At the very least the trait should lose its lifetime and the lifetime should move to the associated type. This should greatly reduce all of our for<'a>s

It's unclear to me if something like Self<'a> is possible, we'd be able to lose the associated type with that.