std.container.rbtree: Return range elements by ref

Author: CyberShadow

std/container/rbtree.d

@@ -679,15 +679,15 @@ private struct RBRange(N)
     /**
      * Returns the first element in the range
      */
-    @property Elem front()
+    ref @property Elem front()
     {
         return _begin.value;
     }
 
     /**
      * Returns the last element in the range
      */
-    @property Elem back()
+    ref @property Elem back()
     {
         return _end.prev.value;
     }