message: subscriber: make `Layer` impls for ` Box<dyn Layer<..>>`  `Send+Sync`
  And for Arc
  This is a fixup for
diff: @@ -957,7 +957,7 @@ where
  subscriber_impl_body! {}
  }
  -impl<C> Subscribe<C> for Arc<dyn Subscribe<C>>
  +impl<C> Subscribe<C> for Arc<dyn Subscribe<C> + Send + Sync>
  where
  C: Collect,
  {
  @@ -972,7 +972,7 @@ where
  subscriber_impl_body! {}
  }
  -impl<C> Subscribe<C> for Box<dyn Subscribe<C>>
  +impl<C> Subscribe<C> for Box<dyn Subscribe<C> + Send + Sync>
  where
  C: Collect,
  {
  