Remove some unnecessary clones.

I found these by grepping for `&[a-z_\.]*\.clone()`, i.e. expressions
like `&a.b.clone()`, which are sometimes unnecessary clones, and also
looking at clones nearby to cases like that.

Author: nnethercote

alloc/src/collections/btree/set.rs

@@ -139,7 +139,7 @@ pub struct Iter<'a, T: 'a> {
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_tuple("Iter").field(&self.iter.clone()).finish()
+        f.debug_tuple("Iter").field(&self.iter).finish()
     }
 }