Bitcraft request: allow using enums as index keys

[bfops]

The ability to use enums as index keys was removed before 1.0, we would like to add it back soon but this may require some design.

cc @jdetter

[gefjon]

• We want specifically to allow "c-style" or "simple" enums, i.e. those where all variants are unit.
• We want to support filtering by these values in all of Rust, C# and TypeScript both in servers and clients.