Proto/Michelson: ensure completeness of type equality

Context

Avoid "catch-all" pattern matching for type comparison functions. This ensures that new constructors to 'a ty/'a comparable_ty must be added to ty_eq/comparable_ty_eq, or the compile complains.

Manually testing the MR

C'mon, it's OCaml!

Checklist

• [ ] Document the interface of any function added or modified (see the coding guidelines)
• [ ] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [ ] For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR