message: Add a Trace impl for usize
  Summary: Useful to have, since usize is quite a common type.
diff: @@ -149,6 +149,10 @@ unsafe impl<'v> Trace<'v> for String {
  fn trace(&mut self, _tracer: &Tracer<'v>) {}
  }
  +unsafe impl<'v> Trace<'v> for usize {
  + fn trace(&mut self, _tracer: &Tracer<'v>) {}
  +}
  +
  unsafe impl<'v> Trace<'v> for i32 {
  fn trace(&mut self, _tracer: &Tracer<'v>) {}
  }
  