Payload-less flexible enums?

It might be good to let users declare that a flexible enum will never have any data-carrying variants. If flexible enums are encoded as `tag, offset_to_variant`, then payload-less enums can be represented just with the tag.

The issue names no files, tests, or entry points. Start by locating the flexible-enum declaration and encoding logic, then clarify how a payload-less declaration should be represented. Done means the declaration and tag-only representation are specified and covered by relevant tests.

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