new type: 1-byte bit mask type

Hi,

I finally finished my 1-byte bitmask type. You can even use it in btree
indexes now. It provides all the standard bit operations (& | ^ ~ << >>)
and I hope someone will find it useful. If anybody has any suggestions
for improvements or questions, please let me know. If it passes muster,
perhaps it could go into the contrib area of one of the next releases.

Oh, and you will note that the or-operator is defined as '||' instead of
'|' -- this is because postgres would not accept a single '|' as an
operator. This will hopefully change at some point in the future.

Adriaan