Make Excluded struct public for generic upsert queries

[LucaCappelletti94]

Backend

PostgreSQL, MySQL, SQLite, Others

What do you want to add to diesel?

At this time, Excluded is de-facto a private struct, and therefore while the excluded method is public, it is not really possible to use it in blanket implementations.

Implementation notes

Just re-export Excluded, or, alternatively, create a trait ExcludedColumn for columns which has an associated type Excluded, which would have the same functionality as having the Excluded struct.

Additional details

No response

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.