support for unnest with multiple args

[zachmu]

select unnest(ARRAY[1, 2], ARRAY[10, 20]);
ERROR:  function unnest(integer[], integer[]) does not exist

This is used by Django on a standard migration path, so it's high priority. Django uses it in an insert like this:

INSERT INTO bug11 (a, b) SELECT * FROM UNNEST(ARRAY[1, 2], ARRAY[10, 20]);

[zachmu]

Partial implementation here:

#2239