message: Safer Arena::iter_chunk
  Summary:
  `or_forward.header` is a union field, it is less safe than use `unpack` result.
  Minor code cleanup.
diff: @@ -275,9 +275,7 @@ impl Arena {
  while p < end {
  let or_forward = &*(p as *const AValueOrForward);
  match or_forward.unpack() {
  - Either::Left(_ptr) => {
  - f(&or_forward.header);
  - }
  + Either::Left(ptr) => f(ptr),
  Either::Right(_forward) => {}
  };
  let n = or_forward.alloc_size();
  