`Defer[Schema]`, `Invariant[Schema]` instances

We should be able to provide a `cats.Defer` instance - this would enable usage of `Defer[Schema].fix`, which would be convenient for recursion (no `lazy val`, a little harder to mess up).

Edit: of course I didn't check it all the way through, `recursive` is exactly like `fix`. Still think the instance might be helpful, though.

Also, `Invariant[Schema]`.