Caching for stable expressions with constant arguments v6

Hi list,

Here's v6 of my expression caching patch. The only change in v6 is
added expression cost estimation in costsize.c. I'm setting per-tuple
cost of CacheExpr to 0 and moving sub-expression tuple costs into the
startup cost.

As always, this work is also available from my Github "cache" branch:
https://github.com/intgr/postgres/commits/cache

This patch was marked as "Returned with Feedback" from the 2011-11
commitfest. I expected to get to tweak the patch in response to
feedback before posting to the next commitfest. But said feedback
didn't arrive, and with me being on vacation, I missed the 2012-01 CF
deadline. :(

I will add it to the 2012-01 commitfest now, I hope that's OK. If not,
feel free to remove it and I'll put it in 2012-Next.

PS: Jaime, have you had a chance to look at the patch? Even if you're
not done with the review, I'd be glad to get some comments earlier.
And thanks for reviewing.

Regards,
Marti