Event Emitter

Implement a class EventEmitter with four methods:

• on(event, listener): register a listener for event. Returns the emitter so calls can be chained.
• off(event, listener): remove one registration of that listener. Removing a listener that is not registered does nothing. Returns the emitter.
• once(event, listener): like on, but the listener runs at most once. off(event, listener) with the original function must also cancel a once registration that has not fired. Returns the emitter.
• emit(event, ...args): call every listener of event in registration order, passing args. Returns true if there was at least one listener, otherwise false.

Edge rules: a listener that removes itself (or others) while emit is running must not cause listeners to be skipped in that same emit. The same function registered twice runs twice.