Context

The type and functions added in this MR are going to be used for the switch to skip_list inbox.

I splitted the code here because I want to have reviews only on this specific part and make sure the proof logic is sound.

Manually testing the MR

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