Proto/Scoru: add a separate functor for history cache in Inbox

Context

This MR moves all code that implements bounded history remembering in/for Scoru inbox to a separate functor (in file bounded_history_repr.ml). The main is to use the same code in Dal, where we need to remember recent skip lists of attested slots to be able to generate refutation proofs involving Dals.

TODO: clean and document bounded_history_repr.ml(i)

Manually testing the MR

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