message: Slightly better impl Trace for Cell
diff: @@ -66,9 +66,7 @@ unsafe impl<'v, T: Trace<'v>> Trace<'v> for RefCell<T> {
  unsafe impl<'v, T: Trace<'v> + Copy> Trace<'v> for Cell<T> {
  fn trace(&mut self, tracer: &Tracer<'v>) {
  - let mut v = self.get();
  - v.trace(tracer);
  - self.set(v)
  + self.get_mut().trace(tracer);
  }
  }
  