refactor: use zero-variants enum for types that are RegisterSet

[SteveLauC]

What does this PR do

For those types implementing trait RegisterSet, defining them with zero-variants enum rather than unit struct as we don't want our users to construct them, and an enum with 0 variants will never be constructed, so using it is more suitable than using unit struct here.

For more discussion, see this thread.

This is not an API change unless users are constructing those unit struct types, which should have no use cases, so no CHANGELOG needed.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API