Kinded subtyping, better file structure, more setoid stuff, reorganize imports.

By moving the later further to the outside, we can kill many occurences
of `Next`, and get rid of laters/except_0s at awkward positions. Also we
can have a sensible eliminator `proto_elim`.

Added LTyCopy typeclasses for products and sums
Removed mutable ref to weak ref subtyping as a consequence.