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?

CHANGELOG.md

@@ -32,10 +32,10 @@ Coq 8.19 is newly supported by this version of std++.
   `{[ (x, y) : nat * nat | x = y ]}`. (by Thibaut Pérami)
 - Add `inv select` and `inversion select` tactics that allow selecting the
   to-be-inverted hypothesis with a pattern.
-- The new `coq-stdpp-bitvector` package contains a library for
-  manipulating bitvectors. Users of the previous unstable version need
-  to change the import path from `stdpp.unstable.bitvector` to
-  `stdpp.bitvector.definitions` and from
+- The new `coq-stdpp-bitvector` package contains a library for `n`-bit
+  bitvectors (i.e., fixed-size integers with `n` bits). Users of the
+  previous unstable version need to change the import path from
+  `stdpp.unstable.bitvector` to `stdpp.bitvector.definitions` and from
   `stdpp.unstable.bitvector_tactics` to `stdpp.bitvector.tactics`. The
   complete library can be imported with `stdpp.bitvector.bitvector`.