message: add is_empty
diff: @@ -289,6 +289,10 @@ impl Tree {
  pub fn root_node(&self) -> Result<IndexedNode, Error> {
  self.resolve_link(self.root)
  }
  +
  + pub fn is_empty(&self) -> bool {
  + self.stored_count == 0
  + }
  }
  /// Reference to the node with link attached.
  