Function fibonacci

ch2-patterns/worker.js:15–20

(limit)

    return sum;
  },
  async fibonacci(limit) {
    await sleep(Math.random() * 100);
    let prev = 1n, next = 0n, swap;
    while (limit) { swap = prev; prev = prev + next; next = swap; limit--; }
    return String(next); // <4>
  },
  async bad() {
    await sleep(Math.random() * 10);
    throw new Error('oh no');