Support injectivity annotations (4.12)

[emillon]

Injectivity annotations are a new feature in 4.12:

• #9500, #9727: Injectivity annotations
  One can now mark type parameters as injective, which is useful for
  abstract types:
  module Vec : sig type !'a t end = struct type 'a t = 'a array end
  On non-abstract types, this can be used to check the injectivity of
  parameters. Since all parameters of record and sum types are by definition
  injective, this only makes sense for type abbreviations:
  type !'a t = 'a list
  (Jacques Garrigue, review by Jeremy Yallop and Leo White)

ocaml/ocaml#9500
ocaml/ocaml#9727