Event emitter

Implement an EventEmitter class. Code registers listeners (callback functions) for named events, and emit calls them.

• on(event, listener) registers listener for event. Registering the same function twice means it runs twice.
• once(event, listener) registers a listener that is removed just before its first call.
• off(event, listener) removes one registration of listener for event: the earliest one, whether it came from on or once. Removing something that isn't registered does nothing.
• emit(event, ...args) calls every listener for event with args, in the order they were registered. It returns true if the event had at least one listener and false otherwise.
• listenerCount(event) returns how many registrations event has.
• on, once and off return the emitter, so calls can be chained.

Changes during an emit: an emit calls exactly the listeners registered when it started. A listener removed by an earlier listener in the same emit still runs this time. A listener added during the emit first runs on the next emit. A once listener is removed before it runs, so if it emits the same event again, it doesn't run a second time.