Update and simplify fixed constants

Context

Protocol H adds two "fixed constants" that are missing from the encoding. This encoding is only used for exposing the constants in the all RPC, so the record type was useless and misleading; it has been simplified.

It's not a drama if this MR is not merged in H.

Manually testing the MR

Covered by tezt regression tests. See updated .out files.

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