events now matches the Node.js 11.12.0 API.

• pass through return value in wrapped emitter.once() listeners

  Now, this works:

  emitter.once('myevent', function () { return 1; });
  var listener = emitter.rawListeners('myevent')[0]
  assert(listener() === 1);

  Previously, listener() would return undefined regardless of the implementation.

  Ported from nodejs/node@acc506c

• Reduce code duplication in listener type check (#67 by @friederbluemle).

• Improve emitter.once() performance in some engines