Fast bignum ldb

I noticed that in code/bignum.lisp, there is some special code for
ldb/dpb on a bignum, that is more intelligent than the current code
which just does a bunch of shifts and ands.  This code is currently
commented out.

The code for ldb was mostly working, and I've fixed it so it works
now.  A simple test of extracting (byte 27 39) from a bignum shows
that it's 4.5 times faster and conses 4 times less.

I've also fixed up the dpb part, but there still appear to be some
issues/confusion on dpb with a byte spec that's bigger than a fixnum,
but the new-byte is a fixnum.

I think, when CVS comes back up, that I'll enable the bignum ldb/dpb
code, but probably will not hook it into the compiler.  That will wait
for some other time.

Ray


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