Check that zippers compose well

[Kerl13]

We know how to generate zippers for two types like

type 'a t1 = (* some constructors *)
type t2 = (* some other constructors *)

But what happens with the type t2 t1?

• If it is defined at the top-level like type foo = t2 t1 @deriving zipper, we should probably generate a new zipper type for this type.
• If not, we should check that the zipper t2 t1_zipper is convenient enough to use