message: Add a Trace instance for Box
  Summary: Useful to have available
diff: @@ -117,6 +117,12 @@ unsafe impl<'v, T: Trace<'v> + Copy> Trace<'v> for Cell<T> {
  }
  }
  +unsafe impl<'v, T: Trace<'v>> Trace<'v> for Box<T> {
  + fn trace(&mut self, tracer: &Tracer<'v>) {
  + Box::as_mut(self).trace(tracer)
  + }
  +}
  +
  unsafe impl<'v, T1: Trace<'v>, T2: Trace<'v>> Trace<'v> for (T1, T2) {
  fn trace(&mut self, tracer: &Tracer<'v>) {
  self.0.trace(tracer);
  