replace retain_mut usage

src/logic/future_moves.rs

@@ -342,11 +342,10 @@ impl FutureMoves {
                     }
                 }
 
-                node.children.retain_mut(|c| {
+                for c in node.children.as_mut_slice() {
                     debug_assert!(index_map.get(*c).unwrap().is_some());
                     *c = unsafe { index_map.get_unchecked(*c).unwrap_unchecked() };
-                    true
-                });
+                }
 
                 node
             })