Tx_rollup,proto: Rejection logic

Context

This implements the logic of rejection -- that is, everything up to the actual verification of the proof. That means that the rejection operation must pass enough information to reconstruct the last agreed message, and that information must be checked. We also check that the commitment is present and not already final.

Manually testing the MR

Test coverage is approaching a reasonable level.

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