Proto: Add missing mli for local-gas-counter module

Context

Some prep work for this issue.

The module Local_gas_counter is lacking an .mli file. This MR introduces an signature file that:

• Makes the type local_gas_counter distinguished from int
• Makes outdated_context abstract.
• Minor changes to Script_interpreter.Internals to avoid reintroducing new types.
• Renames some function to be more intuitive

The rest is accommodating for the API changes.

Manually testing the MR

No additional tests introduced.

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR