Proto/Michelson: fix injectivity of types

Because some type parameters of the type ty are abstract, the OCaml typechecker cannot prove that some types are different, e.g. z num and Tez.t. This leads to impossible cases in pattern matching that have to be covered with assert false. This is dangerous because we may forget to treat new cases when adding new types.

A solution is to make sure OCaml typechecker can distinguish all types used as type parameter of ty. A simple but unfortunately syntactically a bit heavy is to box them with a public constructor, e.g. type tez = Tez of Tez.t [@@ocaml.unboxed] (the annotation allow to have a zero runtime and memory cost)