doubly-linked-list: Allow should_implement_trait

exercises/doubly-linked-list/example.rs

@@ -139,6 +139,7 @@ impl<T> Cursor<'_, T> {
         }
     }
 
+    #[allow(clippy::should_implement_trait)]
     pub fn next(&mut self) -> Option<&mut T> {
         // safe as node.next is a valid potential pointer
         unsafe { self._step(|node| node.next) }

exercises/doubly-linked-list/src/lib.rs

@@ -44,6 +44,7 @@ impl<T> Cursor<'_, T> {
 
     /// Move one position forward (towards the back) and
     /// return a reference to the new position
+    #[allow(clippy::should_implement_trait)]
     pub fn next(&mut self) -> Option<&mut T> {
         unimplemented!()
     }