Use Keyword views to R/W ref attributes

Make the `ToDRef` type class more flexible: allow types viewable as `Keyword` to be lifted into a `DRef`.
and similarly replace `Attribute2EntityReaderCast[DRef, Card, Keyword]` with `Attribute2EntityReaderCast[DRef, Card, K]` with implicit `Keyword => K`.
Merge branch 'topic/rw-kw-implicits' into maintenance
bump version for next maintenance release
Merge branch 'maintenance' as v0.5.2 release