Support for JSON/JSONB

[KellerFuchs]

Since version 9.2, PostgreSQL has support for native JSON datatypes: json and jsonb (jsonb is a binary representation with more efficient access).

It would be pretty great to be able to manipulate those from Rust PgSQL extensions, though I guess it requires #2 to happen first.

My usecase would be an extension that validates a JSON value against a JSON Schema (itself represented as a JSON value).