Support enums as field and tag values

It would be helpful to use enum values in many cases. Enums can then be converted to/from strings when writing/reading to/from DB.

I have implemented such support. Should I submit a PR?

No files, tests, or entry points are named. Start by locating the existing field and tag serialization and deserialization paths, then inspect their tests. Done should mean enum values can be converted to and from strings when writing to and reading from the database.

Written by the indexing model from the issue text.