Proto/Michelson: move entrypoints out of types

Context

First part of the fifth step to ignoring annotations (tzip!138 (closed), #2025 (closed)): moving entrypoints out of the types so that we can remove annotations from Union_t later. The fourth step was !4175 (merged).

The big change happens in the commit Proto/Michelson: move entrypoints out of types. The following commits take care of getting of the two added assert false. I could probably squash the commits if reviewers want to.

Tests

CI

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, the Development Version section of CHANGES.md for everything else). -> done in !4259 (merged)
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR