BIT/BIT VARYING status

I have made a first cut at completing integration of Adriaan Joubert's
BIT code into the backend. There are a couple little things left to
do (for example, scalarltsel doesn't know what to do with BIT values)
as well as some not-so-little things:

1. SQL92 mentions a bitwise position function, which we do not have.

2. We don't handle <bit string> and <hex string> literals correctly;
the scanner converts them into integers which seems quite at variance
with the spec's semantics.

We could solve #2 fairly easily if we don't mind breaking backwards
compatibility with existing apps that expect B'101' or X'5' to be
equivalent to 5. I'm not sure how to handle it without breaking that
compatibility. Thoughts?

regards, tom lane