Proto/Michelson: remove comparable_ty smart constructors

Context

Follows !4502 (merged) and !4721 (merged).

Remove ..._key functions in favour of ..._t ones.

Rename pair/union/option_key to comparable_pair/union/option_t.

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