proposed todo: use insert/update returning anywhere a table is allowed

It would be great to be able to join to update, returning, etc. It
looks like the code was deliberately tied off as-is and I was
surprised not to see a todo for this.

the basic idea is to be able to do things like:
select * from (update foo set id = 1 returning *) q;

which currently syntax errors out on 'set' which is a bit odd.

here are tom's brief notes on it and also some context on how select
combined with insert/update might influence triggers:

http://archives.postgresql.org/pgsql-hackers/2006-10/msg00693.php

merlin