Proto/AI: simplify misbehaviour_repr

based on !11898 (merged)

What

Simplify Misbehaviour_repr.t and expose some conversion functions in Internal_for_tests modules.

Why

This will makes testing this code easier, see !11601 (merged).

How

• The slot field is unused so we remove it.
• Merge types Misbehaviour_repr.kind and Validate_errors.Anonymous.denunciation_kind, which do the same thing.
• Expose functions to convert raw_level and round between versions below/above the Alpha_context abstraction.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• [N/A] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [N/A] 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).