Three more examples of monotonicity failures in OCaml

[yallop]

This PR adds three more examples of monotonicity failures in OCaml, relating to

1. the relaxed value restriction and strict positivity
2. compatibility
3. labeled arguments

Another possible addition involves constructor disambiguation:

type t = A | B

module M :
sig
  type t (* = A *)
end =
struct
  type t = A
end

let f = M.[A;B]

but I think that one's less convincing, because it's more about namespacing than type systems as such.