revised patch for PL/PgSQL table functions

This is an improved version of the PL/PgSQL table function
patch. Since the last patch, I've implemented support for returning
ROWTYPE variables and scalar expressions under RETURN NEXT, changed
the grammar a little bit, and added some regression tests.

Remaining to be done:

        - write the documentation, add a few more regression tests

        - fix the grammar: I'm still using the 'NEXT' hack instead of
          'RETURN NEXT'. It may also be better to use 'RETURN xxx AND
          RESUME' instead.

        - memory management: stop using TopTransactionContext. This
          isn't that urgent, IMHO.

Cheers,

Neil

--
Neil Conway <neilc@samurai.com> || PGP Key ID: DB3C29FC