remove clone in dlist_arc iter_immut

ghostcell/src/dlist_arc.rs

@@ -102,11 +102,11 @@ impl<'id, T> Node<'id, T> {
         token: &GhostToken<'id>,
         f: impl Fn(&T),
     ) {
-        let mut cur: Option<NodePtr<'id, T>> = Some(node.clone());
+        let mut cur: Option<&GhostCell<'id, Node<'id, T>>> = Some(node.as_ref());
         while let Some(node) = cur {
             let node: &Node<'id, T> = node.borrow(token); // immutably borrow `node` with `token`
             f(&node.data);
-            cur = node.next.clone();
+            cur = node.next.as_deref();
         }
     }
 }