pl/python invalidate functions with composite arguments

Here's a patch implementing properly invalidating functions that have
composite type arguments after the type changes, as 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/invalidate-composites.

The idea in general is for this to work (taken straight from the unit
tests, btw)

CREATE TABLE employee (
    name text,
    basesalary integer,
    bonus integer
);
INSERT INTO employee VALUES ('John', 100, 10), ('Mary', 200, 10);

CREATE OR REPLACE FUNCTION test_composite_table_input(e employee)
RETURNS integer AS $$
return e['basesalary'] + e['bonus']
$$ LANGUAGE plpythonu;

SELECT name, test_composite_table_input(employee.*) FROM employee;
ALTER TABLE employee DROP bonus;
-- this fails
SELECT name, test_composite_table_input(employee.*) FROM employee;
ALTER TABLE employee ADD bonus integer;
UPDATE employee SET bonus = 10;
-- this works again
SELECT name, test_composite_table_input(employee.*) FROM employee;

It's a long-standing TODO item, and a generally good thing to do.

Cheers,
Jan