Support factors

[multimeric]

I know this is a known issue, but it I feel like it could do with an issue for tracking purposes.

Replication:

> duckplyr:::duckdb_rel_from_df(iris)
Error in duckplyr:::duckdb_rel_from_df(iris) : 
  Can't convert factor columns to relational. Affected column: `Species`.

[grantmcdermott]

Factor support could be implemented via enum types, no? Or, am I missing something obvious?

[BlackEdder]

Doing some digging it looks like there was a missing feature in duckdb, which made it complicated, but that bug is now solved! Hopefully factors can be enabled soon.

[krlmlr]

While we can add support for factors, enabling this will almost certainly unlock test failures that were hidden before because we had a fallback as soon as factors were present.

Action items:

• Enable
• Fix internal tests
• Run revdepchecks, fix