Support `returns Table(x type, y type)` custom functions

Right now, functions must return a `setof ` such as:

```
CREATE FUNCTION good_things() RETURNS SETOF things AS $$
SELECT * FROM things WHERE good = true;
$$ LANGUAGE sql STABLE;
```

The problem is that the table `things` needs to already exist. Would be great if you could also specify a dynamic table as a return type too, such as:

```
CREATE FUNCTION better_things() RETURNS TABLE (key int, info text, retrieved_at timestamptz) AS $$
SELECT id as key, data as info, now() as retrieved_at FROM things WHERE good = true;
$$ LANGUAGE sql STABLE;
```

Doing so would add flexibility on the type of data returned, such as adding additional columns or hiding other columns. Furthermore, the `RETURNS TABLE` option provides an easy way to work with `RETURN NEXT`:

```
CREATE FUNCTION the_best_things() RETURNS TABLE (key int, info text, retrieved_at timestamptz) AS $$
BEGIN
FOR thing IN SELECT * FROM things WHERE good = true LOOP
-- You can do some imperative stuff in this block now.
key := thing.id;
info := thing.data;
retrieved_at := now();

PERFORM pg_notify('thing-' || key, 'retrieved_at: ' || extract(epoch from retrieved_at));

RETURN NEXT;
END LOOP;
END;
$$ LANGUAGE plpgsql STABLE;
```

Interested in any thoughts.

The issue names no files, tests, or entry points. Start by locating the PostgreSQL custom-function support and its handling of SETOF return types; done should mean accepting RETURNS TABLE definitions and making their declared columns available through the resulting API.

Written by the indexing model from the issue text.