rust: Improve Rust bindings

The big change here is that all API calls now take a &Handle
rather than making a copy. This, apart from being more sensible
and efficient, allows us to implment Drop on the handle so that
it will call _free() when it goes out of scope.

There's some jiggery-pokery with a clone flag in there now
because of callbacks that can return a valid handle, and we
want those to be Drop'ed sensibly.