subselects

OK, I have added code to allow the SubLinks make it to the optimizer.

I implemented ParseState->parentParseState, but not parentQuery, because
the parentParseState is much more valuable to me, and Vadim thought it
might be useful, but was not positive.  Also, keeping that parentQuery
pointer valid through rewrite may be difficult, so I dropped it. 
ParseState is only valid in the parser.

I have not done:

	correlated subquery column references
	added Var->sublevels_up
	gotten this to work in the rewrite system
	have not added full CopyNode support

I will address these in the next few days.

-- 
Bruce Momjian
maillist@candle.pha.pa.us