Update of bitmask type

Hi,

here is a new version of the bitmask type. It supports hash-indices as
well now, and fixes a bug in the definition of the <> operator.

I would appreciate it if somebody more knowledgable than myself would
look over the index definitions. They seem to work and are used by
postgres, so I guess they can't be all wrong. The hashing function is
the same as that for char's and comes straight out of the postgres
source code.

BTW, chapter 36 of the documentation could do with some additions, but I
don't feel knowledgable enough to attempt it. E.g. it shows how to put
an entry for the hashing into pg_amop, but never explains how to define
the entry in pg_amproc and doesn't tell you that you need to define a
separate hashing function. It took me a while of looking through the
other definitions and digging through the source code to come up with a
best guess.

Perhaps this could go into the contrib area if it passes muster, as it
is an example of a user-defined type with indices.

Cheers,

Adriaan