Lib_micheline: allow primitives starting with _

Context

This MR extends the Micheline lexer to allow primitives starting with the underscore symbol (_). In particular, this allows to consider the string "_" as a valid primitive. This has no impact on Michelson because, in each protocol, Michelson uses a fixed set of primitives (none of which starting with "_") but this is useful for extensions using "_" as a pattern such as TZT.

Manually testing the MR

dune build @src/lib_micheline/runtest

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