There are minor benefits to using fxvectors instead of vectors:
the GC knows that an fxvector has no pointers inside, and the
bytecode compiler knows that `fxvector-ref' produces a fixnum.
BTW, a benefit of fxvectors over byte strings is the lack of an
indirection internally. (Unlike a byte string, the content of
an fxvector is "inlined" into the vector object.)
Change `bit-vector-count' to `bit-vector-length', add arguments
to `bit-vector-copy', use `racket/private/vector-wraps' (which
should be moved to a public place) to implement things like
`for/bit-vector'.
