Proto: make Destination a polymorphic variant (SCORU)

Context

This MR transforms Contract.t variant into a polymorphic variant, as well as Destination.t, now defined as an extension of Contract.t rather than a wrapper (yummy subtyping!). It also exposes the constructors of Contract.t which didn't need to be abstract.

This is as first step towards: adding new destinations and specializing internal operations, both wanted for SCORU.

Manually testing the MR

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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