message: Add more Trace implementations
  Summary: Empty and singleton tuples make sense to have a Trace for.
diff: @@ -122,6 +122,16 @@ unsafe impl<'v, T: Trace<'v> + ?Sized> Trace<'v> for Box<T> {
  }
  }
  +unsafe impl<'v> Trace<'v> for () {
  + fn trace(&mut self, _tracer: &Tracer<'v>) {}
  +}
  +
  +unsafe impl<'v, T1: Trace<'v>> Trace<'v> for (T1,) {
  + fn trace(&mut self, tracer: &Tracer<'v>) {
  + self.0.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);
  