add defaults for the several structs

src/data/arena.rs

@@ -52,10 +52,16 @@ pub struct Index {
     generation: u32,
 }
 
-impl IndexedData for Index {
+impl Default for Index {
     fn default() -> Self {
         Self::from_raw_parts(crate::INVALID_U32, crate::INVALID_U32)
     }
+}
+
+impl IndexedData for Index {
+    fn default() -> Self {
+       Default::default()
+    }
 
     fn index(&self) -> usize {
         self.into_raw_parts().0 as usize

src/data/graph.rs

@@ -125,7 +125,7 @@ impl<E> Edge<E> {
     }
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug,Default)]
 #[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
 pub struct Graph<N, E> {
     pub(crate) nodes: Vec<Node<N>>,