Support RETURNS TABLE (...) functions

[eeeebbbbrrrr]

This PR gets support for set returning functions. It needs tests and another once over, but this little sql works now:

[v15.3][483091] plrust=# create function foo() returns table (a int, b int) language plrust as $$

Ok(Some(TableIterator::new(vec![(Some(1), Some(2))].into_iter())))
$$;
CREATE FUNCTION
Time: 3683.713 ms (00:03.684)
[v15.3][483091] plrust=# select * from foo();
 a | b 
---+---
 1 | 2
(1 row)

[BradyBonnette]

Looks good, if more changes are needed lmk!

[eeeebbbbrrrr]

I'm gonna hold this until we get pgcentralfoundation/pgrx#1164 over in pgrx merged and released.

This PR will need a test around returning a table with 1 field, which is blocked on that pgrx PR right now.

[eeeebbbbrrrr]

I'm gonna hold this until we get tcdi/pgrx#1164 over in pgrx merged and released.

This PR will need a test around returning a table with 1 field, which is blocked on that pgrx PR right now.

Done. Added the dependency updates to this PR too, since it needs them. Once CI passes I'm gonna merge.