Proto: add Make_carbonated_map.keys_unaccounted.

Context

Partly answers #3291 (closed).

It adds a keys_unaccounted function that lists keys of a carbonated map without accounting for gas. It will be used when PVM_kind is carbonated and we don't have access to PVM_kind.keys anymore.

Manually testing the MR

The new function will be tested when used in PVM_kind, in !5813 (merged).

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)
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR