Create casts from meta-identifiers to OIDs

[erichanson]

We've excluded PostgreSQL oids from the catalog intentionally, but sometimes they are needed. Thought about adding an oid column to the appropriate meta.* views, but it really departs from meta's "above-the-hood" philosophy.

Instead, make a cast from meta-identifiers to oids. For example:

select meta.relation_id('a','b')::oid;