tree: add OID->OID cast map

This commit adds a new map that describes valid casts from OID to OID.
It introduces three cast contexts: explicit casts, assignment casts, and
implicit casts. See the comments for CastContext and castMap for
details.

This map will enable us to properly follow Postgres's casting behavior.
Most immediately, it will allow us to support assignment casts.

Future work includes moving volatility in castMap. In the longer term,
cast functions can be moved into castMap as well.

Release note: None