BUG #1578: ::bit(n) behaves "differently" if applied to bit strings than integers.

The following bug has been logged online:

Bug reference:      1578
Logged by:          Celia McInnis
Email address:      celia@drmath.ca
PostgreSQL version: 8.0.1
Operating system:   Linux
Description:        ::bit(n) behaves "differently" if applied to bit strings
than integers.
Details:

It's probably not good (at least for mathematicians!) to have the following
give different results:

select B'1110110101'::bit(6);
select B'1110110101'::integer::bit(6);

The first gives 110101 (the 6 least significant bits).
The second gives 111011 (the 6 most significant bis).