Event Emitter (Pub/Sub)

Decouple parts of an app with events: the code that emits "user_signed_up" doesn't need to know who's listening.

Implement a class EventEmitter:

• on(event, fn): register a listener and return fn, so callers can keep it to pass to off later.
• once(event, fn): a listener that runs at most once.
• off(event, fn): remove a listener. Removing one that isn't registered does nothing.
• emit(event, *args, **kwargs): call listeners in registration order and return how many were called.
  • Listeners added during an emit don't run in that emit.
  • If a listener raises, still call the rest, then raise the first exception.