message: Implement `Widget::tag` for `pure::Canvas`
diff: @@ -105,6 +105,10 @@ where
  P: Program,
  B: Backend,
  {
  + fn tag(&self) -> tree::Tag {
  + tree::Tag::of::<P::State>()
  + }
  +
  fn state(&self) -> tree::State {
  tree::State::new(P::State::default())
  }
  