pl/python table functions

Here's a patch implementing table functions mentioned in
http://archives.postgresql.org/pgsql-hackers/2010-12/msg01991.php. It's
an incremental patch on top of the plpython-refactor patch sent eariler.

Git branch for this patch:
https://github.com/wulczer/postgres/tree/table-functions.

This allows functions with multiple OUT parameters returning both one or
multiple records (RECORD or SETOF RECORD). There's one inconvenience,
which is that if you return a record that has fields of composite types,
the I/O functions for these types will be looked up on each execution.
Changing that would require some juggling of the PL/Python structures,
so I just left it at that.

Note that returning just the composite type (or a set of them) does
cache the I/O funcs. You get the repeated lookups only if the function
returns an unnamed record, that has composite field among others, so
something like

CREATE FUNCTION x(OUT x table_type, OUT y integer) RETURNS RECORD

which I think is fairly uncommon.

Cheers,
Jan