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 maintained by Michael Sammler. *)
+From stdpp.bitvector Require Export definitions tactics.
+From stdpp Require Import options.