[PATCH] bitmap indexes

Hi.

This is a cleaned-up and rebased version of the bitmap index patch from
Gavin Sherry, later revised by Gianni Ciolli and Gabriele Bartolini, and
others including Daniel Bausch.

I've been working on this patch for a while, and have made some progress
towards (a) general fixing, and (b) a working VACUUM implementation (the
major remaining piece). Unfortunately, I've been busy moving house, and
the latter is not complete (and not in this patch).

I will continue working on the code, and I'll post updates. I expect to
have more to show in just a few days.

Nevertheless, I'm posting it for review now as I keep working. Given the
size and age of the patch, I would appreciate any comments, no matter
how nitpicky.

Thanks.

-- Abhijit

P.S. There are some brokennesses, marked with XXXes in the code. See
also http://www.postgresql.org/message-id/20081101000154.GO27872@fune
and bmi-perf-test.tar.gz in particular.