jsonb generation functions

In the jsonb patch I have been working on, I have replicated all of what
I call the json processing functions, and I will shortly add analogs for
the new functions in that category json_to_record and json_to_recordset.

However I have not replicated what I call the json generation functions,
array_to_json, row_to_json, to_json, and the new functions
json_build_array, json_build_object, and json_object, nor the aggregate
functions json_agg and the new json_object_agg. The reason for that is
that I have always used those for constructing json given to the client,
rather than json stored in the database, and for such a use there would
be no point in turning it into jsonb rather than generating the json
string directly.

However, I could be persuaded that we should have a jsonb analog of
every json function. If we decide that, the next question is whether we
have to have it now, or if it can wait.

(The other notable thing that's missing, and I think can't wait, is
casts from json to jsonb and vice versa. I'm going to work on that
immediately.)

cheers

andrew