Updated bitmap index patch

Hi all,

I've updated the bitmap index patch. It can be found here:

http://www.alcove.com.au/~swm/bitmap-2007-03-12.diff

This fixes some bugs introduced in the last patch, catches up to HEAD and
tidies up the executor code. I want to do a little more tidying, such as
reverting the name changes we made. multiscan still sounds alright.

I've been thinking about vacuum as well. Something along the lines of what
Heikki mentioned earlier -- namely, the ability to iterate the bitmap
setbit by setbit is in order. What I have in mind, though, is that when we
find a reaped setbit, we mark the position and continue to iterate until
we find a non reaped setbit. When, we update the underlying bitmap vector
to reflect the non-set bits.

Thoughts?

Thanks,

Gavin