def unshift(node)
      node = Node(node)
      @tail ||= node

      node.next = @head
      @head.prev = node if @head
      @head = node

      @length += 1
      self
    end
def unshift(node)
      node = Node(node)
      @tail ||= node

      node.next = @head
      @head.prev = node if @head
      @head = node

      @length += 1
      self
    end
Pushes new nodes on top of the list.

 == Parameters:
 node:: Any object, including +Node+ objects.

 == Returns:
 +self+ of +List+ object.