Add several generator functions for jsonb that exist for json.

The functions are:
    to_jsonb()
    jsonb_object()
    jsonb_build_object()
    jsonb_build_array()
    jsonb_agg()
    jsonb_object_agg()

Also along the way some better logic is implemented in
json_categorize_type() to match that in the newly implemented
jsonb_categorize_type().

Andrew Dunstan, reviewed by Pavel Stehule and Alvaro Herrera.