Proto/Michelson: GADTify stuff

Context

Preparatory work for !4502 (merged). A new type parameter will be added to ty. It will require to be existentially or universally quantified (depending on the places).

The goal of this MR is to reduce the size of future diffs.

MR based on !4578 (merged)

Manually testing the MR

🐫

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