Refactor to split nodeAgg.c?

I was going to rebase my HashAgg patch, and got some conflicts related
to the grouping sets patch. I could probably sort them out, but I think
that may be the tipping point where we want to break up nodeAgg.c into
nodeSortedAgg.c and nodeHashAgg.c, and probably a common file as well.

This would also (I hope) be convenient for Simon and David Rowley, who
have been hacking on aggregates in general.

Anyone see a reason I shouldn't give this a try?

Regards,Jeff Davis