JSON generation improvements.

This adds the following:

    json_agg(anyrecord) -> json
    to_json(any) -> json
    hstore_to_json(hstore) -> json (also used as a cast)
    hstore_to_json_loose(hstore) -> json

The last provides heuristic treatment of numbers and booleans.

Also, in json generation, if any non-builtin type has a cast to json,
that function is used instead of the type's output function.

Andrew Dunstan, reviewed by Steve Singer.

Catalog version bumped.