Updated bitmap index patch

Hi all,

Attached is an updated bitmap index patch. It contains bug fixes, API
changes, binary changes (page identifier to distinguish it from other
indexes) and has been brought up to HEAD.

I worked on a few approaches to VACUUM, none very satisfactory. The
problem is, breaking a compressed word representing matches can have
serious consequences -- at the least, creation of new words, at the worst,
creation of a new page. If a lot of this were to happen, REINDEX would be
much more efficient (this is what earlier patches did).

One approach I looked at was modifying the existing read API to be able to
do something like "kill prior tuple". This, I think, made the API quite
complex and it was hard to implement, since the existing mechanism
decompresses words on the fly and it would be hard to identify which TID
is no longer a match. So, I dropped this idea pretty quickly.

The second approach is to just manually traverse each vector and change
matches to non-matches where necessary. The complexity then is in managing
the consequences of breaking compressed words, doing WAL (efficiently) and
calculating free space. I've only partially implemented this approach. At
this stage, I don't have time to finish it due to other commitments.

Thanks,

Gavin