[Bizgres-general] On-disk bitmap index now applied

[Luke Lonergan]

I've just applied Jie Zhang's on-disk bitmap index to the Bizgres CVS.  This
version has the new ability to bypass the in-memory bitmap index formation
and perform AND and OR operations directly from the on-disk bitmap
representations, which should make it perform better.  Also, the new bitmap
compression scheme results in on-disk index sizes that are far smaller than
BTREE.

Please test and report results!

- Luke