put(key, value) {
    if (super.has(key)) super.delete(key);
    super.set(key, value);
    if (super.size > this.capacity) {
      const oldestKey = super.keys().next().value;
      super.delete(oldestKey);
    }
  }
Upsert key/value pair. Updates mark keys are recently used.
@param {number} key
@param {number} value
@returns {void}