first-draft patch for aggregate ORDER BY

What it does:

# select array_agg(b order by a)
from (values (3,'foo'),(2,'bar'),(1,'baz')) v(a,b);
array_agg
---------------
{baz,bar,foo}
(1 row)

What it doesn't do:

- no docs or regression tests yet

- no support for agg(... ORDER BY ...) OVER window
(which the spec does allow)

Requires initdb. Beware of bugs. Slippery when wet.

--
Andrew (irc:RhodiumToad)