message: add function to determine if the list is empty
diff: @@ -44,6 +44,10 @@ impl<T> DoublyLinkedList<T> {
  }
  }
  + pub fn is_empty(&self) -> bool {
  + self.head.is_none()
  + }
  +
  pub fn head(&self) -> Option<Rc<RefCell<Node<T>>>> {
  self.head.as_ref().cloned()
  }
  