Proto/Michelson: add the UNIT instruction to the IR

Context

The UNIT instruction is currently handled very specifically; it is a kind of macro which is expanded to PUSH unit Unit at elaboration time.

This MR promotes it to a real instruction by adding a case in the GADT. The only observable effect is a slightly reduced footprint in the contract cache for scripts using this instruction.

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