[plt-scheme] srfi-4 u8vectors and srfi-66

There was a problem with srfi-4's (the foreign interface's, actually)
implementation of `u8vector' -- srfi-66 is naturally implemented using
byte strings, but the specification says that it should use u8vectors.
So I changed the implementation to use byte string operations for
u8vectors.

The only difference is visible when passing a vector to C --
(make-u8vector 0) is now the same as (make-bytes 0), which produces a
useless vector.  Things like (make-s8vector 0) will translate to NULL
when passed to C.

Please tell me if you see a problem with this.
-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!