keys and items shouldn't take a mutable reference to self.

src/storage.rs

@@ -35,7 +35,7 @@ impl Storage {
         &mut self.items[id]
     }
 
-    pub fn items(&mut self) -> impl Iterator<Item = &Item> {
+    pub fn items(&self) -> impl Iterator<Item = &Item> {
         self.items.iter()
     }
 
@@ -43,7 +43,7 @@ impl Storage {
         self.items.iter_mut()
     }
 
-    pub fn keys(&mut self) -> impl Iterator<Item = &String> {
+    pub fn keys(&self) -> impl Iterator<Item = &String> {
         self.keys.keys()
     }
 }