Export BitVec so a key type can be declared outside the crate

[arthaud]

The patricia tree containers are keyed by K: Into<BitVec>, and their iterators want K: From<&BitVec>. BitVec lives in a private module that datatype never re-exports, so no crate outside this one can write either impl: the bounds are satisfiable only by the integer types implemented here, and a caller keying on a newtype of its own has to fall back to HashMapAbstractEnvironment and give up the structural sharing that is the reason to reach for a patricia tree.

Re-export it, the way every other module in datatype is. Nothing else in bitvec is public, so this exports exactly the one type. Add to_int, the counterpart of from_int_with_len, without which From<&BitVec> can only be written a bit at a time while the conversions here reach for the private bits field; they now go through it instead. Add is_empty, which clippy asks for once the type is public.

Test plan: cargo test -- foreign_key_test.rs declares a key type the way a user of the library has to and exercises it against the map, the set and the environment. Note that main does not build: a30d887 calls update_node_by_key with an op taking its node by value, which b033084 had changed to take it by reference. This branch is based on main and was verified on b033084.