Manifest: some small cleanup regarding lib_protocol

Context

The MR implement some small refactoring and cleanup of the manifest regarding how we deal with protocols.

• it groups number and name under an abstract type Protocol.Name.t allowing to pass a single value around (instead of name_dash, name_underscore, name, number)
• remove repetition of "src/proto_%s" name_underscore by introducing helper functions
• Simplify some pattern matching on number.
• simplify the link_protocol script

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