Return onCancel callback from the executor

[Filyus]

It seems to me that instead of

executor(resolve, reject, (onCancel) => {
  internals.onCancelList.push(onCancel);
})

const promise = new CancelablePromise((resolve, reject, onCancel) => {
  onCancel(() => {
    //cancel code
  });
});

it is a bit easier to use such code

const onCancel = executor(resolve, reject);
internals.onCancelList.push(onCancel);

const promise = new CancelablePromise((resolve, reject) => {
  return () => {
    //cancel code
  };
});