create new package for stdpp-bitvector library

This is a step towards #204 (closed).

I was not quite sure which Coq logical path to use for this. For now I used stdpp.bv since always typing stdpp.bitvector seems a bit annoying, but the inconsistency may also be confusing. Opinions?

stdpp_bitvector/bitvector.v

-(** This file is still experimental. See its tracking issue
-https://gitlab.mpi-sws.org/iris/stdpp/-/issues/145 for details on remaining
-issues before stabilization. This file is maintained by Michael Sammler. *)
+(** This file is maintained by Michael Sammler. *)
 From stdpp Require Export numbers.
 From stdpp Require Import countable finite.
 From stdpp Require Import options.