Proto/Michelson: simplify merge functions

Context

Second part of the sixth step to ignoring annotations (tzip!138 (closed), #2025 (closed)): simplifying merge functions in the elaborator. We now have the property that two equal types (according to ty_eq/merge_types) are structurally equal. This allows to rely on checking equality rather than merging. Doing so we save a lot of garbage that was created before (this is a redo of the sixth part of !2482 (closed)). The first part of the sixth step was !4297 (merged).

Less garbage means less ...

• gas? @lrand -> #2483 (closed)
• size in cache? @yrg -> !4298 (comment 840880738)

Testing

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).
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