Draft: Proto,Tx_rollup: Rejections

Context

This is the third MR of a planned Merge Request #3.A (see the related document for more information). Changes from !4369 (merged):

• Rejection operation

This is still missing rejection rewards, because in order to know whom to reward, we need prerejections. So that will come in a later MR. And, of course, until we have Merkle proofs, we cannot really know which rejections are valid, so that is waiting on !4086 (merged).

Mostly, this is independent of !4369 (merged), but if I don't put it on top, the merge will be annoying later. And, of course, it's weird to have rejections without a way to create commitments -- if nothing else, it would be impossible to test!

Manually testing the MR

Unit tests!

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