Added copy trait bound

Author: LucaCappelletti94

tuplities-flatten-nest/src/nested_ref.rs

@@ -3,7 +3,7 @@
 /// Trait to obtain a nested tuple of references from a nested tuple of values.
 pub trait NestedTupleRef {
     /// The nested tuple type containing references to each leaf value.
-    type Ref<'a>
+    type Ref<'a>: Copy
     where
         Self: 'a;