code: public function mapKeys(callable $callback)
      {
          $items = [];
  
          foreach ($this->items as $key => $value) {
              $items[$callback($key, $value)] = $value;
          }
  
          return $this->createFrom($items);
      }
docs: Applies the given function to each _key_ in the bag and returns
  a new bag with the keys returned by the function and their values.
  
  @param callable $callback Function is passed `($key, $value)`
  
  @return static