A consistent way to cancel subscriptions from within the subscription

[philpax]

At present, it is very difficult to do one-shot / conditionally-active subscriptions, because you can't refer to the return value of a subscriber from within the closure:

let listener = Frame::subscribe(|_| {
  listener.stop(); // this will not work
});

An idea is to add a Context parameter that includes the message's source and to provide a stop method. We want to avoid using the return value as that may be used for some other form of semantics (i.e. #253 or early-exiting a chain of handlers).