Add id to items method.

src/storage.rs

@@ -42,8 +42,8 @@ impl Storage {
         &mut self.items[id]
     }
 
-    pub fn items(&mut self) -> impl Iterator<Item = &Item> {
-        self.items.iter()
+    pub fn items(&mut self) -> impl Iterator<Item = (usize, &Item)> {
+        self.items.iter().enumerate()
     }
 
     pub fn items_mut(&mut self) -> impl Iterator<Item = &mut Item> {