WIP: grouping sets support

Hello,

there is basic support of GROUPING SETS. This patch isn't ready for
production! It's only material for discus. Parser and executor are
done. Planner part should be completely rewritten. I don't understand
well to planner, and I haven't idea that parts should be shared with
groupby. Implementation is based on using hash tables that holds
result. It implements push mode via new node feeder.

Parser is ugly - there was two problems - 1. collision '(' ... ')'
between set spec and ROW constructor, 2. identificator cube shouldn't
be keyword (compatibility with cube contrib module).

I invite any help or comments. Without help I am not able to prepare
this patch for last commitfest. Help is necessary in two parts -
documentation, planner and testing. This version is based on CVS HEAD
before CTE patch!

knows limits:
* only hash aggregates are supported
* doe's somebody understand what's mean ALL in GROUP BY CLAUSE?

Regards
Pavel Stehule