Draft: Proto,Tx_rollup: Pre-rejections

Context

We want to prevent bakers from taking rejection rewards for rejections computed by others. With this MR, rejections must be preceded by prerejections, and the sender of the first prerejection gets the reward.

Manually testing the MR

The only tests so far are that the rejection tests still work (once they send correct prerejections). Of course, we will have to test various sorts of incorrect prerejections.

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