Currently it's required that you take mut self in order to manipulate
the internal state machines. This would not allow passing an Arc/Rc to
a callback since A/Rc<T> only derefs to &T and not T.

The take_mut crate allows the retrieval of ownership if you have a &mut refference
and as long you return T again. So Arc<Mutex<Machine> could work with
callbacks and embed Nested state machies without copying.