Proto/Scoru: uniformly use Z.t instead of int64 for scoru inboxes fields

Context

This MR changes SCORUs inboxes fields nb_available_messages and nb_messages_in_commitment_period to have them of type Z.t instead of int64. This change:

• makes things uniform, as message_counter (which is supposed to be smaller) is already a Z.t
• surprisingly reduces gas consumption (see newly generated regression test files), because it seems encodings for Z.t take less place (for small numbers) than int64

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