Proto/Michelson: another preparatory work for uniting types

Context

Another preparatory work for !4502 (merged):

• pair and union smart constructors return type must be wrapped in an existential because we won't know whether the result is comparable or not
• option, pair, and union constructors will carry a (GADT) witness of their comparable-ness, hence a new parameter must be added to them

These changes look quite stupid as is but will make later MRs smaller. Except for minor gas/size changes, they should keep the semantics as is.

The type to_be_replaced is removed in the next MR: !4693 (merged)

Manually testing the MR

CI (I may need to update a few constants or regression tests)

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). (will do at the end)
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR