Support primitive collections as JSON (instead of an array)

[roji]

The default - and natural - way to map a primitive collection (e.g. int[]) in PostgreSQL is to a database array (the PG integer[]) type. This is PG-unique feature; in other database, primitive collections are mapped to a JSON text column in the database.

However, in some cases it may still make sense to map primitive collections to JSON in PG, just like we do in other databases. While we seem to support the mapping itself, most forms of querying won't work (e.g. we need to translate to jsonb_to_recordset instead of unnest).

Since mapping to PG arrays is the recommended way, this isn't likely to be high priority. Putting this in the backlog to gather user feedback.