GROUPING SETS revisited

In case anyone's interested, I've taken the CTE-based grouping sets patch from
[1] and made it apply to 9.1, attached. I haven't yet done things like checked
it for whitespace consistency, style conformity, or anything else, but (tuits
permitting) hope to figure out how it works and get it closer to commitability
in some upcoming commitfest.

I mention it here so that if someone else is working on it, we can avoid
duplicated effort, and to see if a CTE-based grouping sets implementation is
really the way we think we want to go.

[1] http://archives.postgresql.org/pgsql-hackers/2009-05/msg00700.php

--
Joshua Tolley / eggyknap
End Point Corporation
http://www.endpoint.com