message: add trait bound for older rust versions
diff: @@ -252,7 +252,7 @@ fn try_match<N, E, Ty, Ix, F, G>(mut st: &mut [Vf2State<Ty, Ix>; 2],
  }
  #[derive(Clone, PartialEq, Debug)]
  - enum Frame<N> {
  + enum Frame<N: marker::Copy> {
  Outer,
  Inner{ nodes: [N; 2], open_list: OpenList },
  Unwind{ nodes: [N; 2], open_list: OpenList },
  @@ -438,7 +438,7 @@ fn try_match<N, E, Ty, Ix, F, G>(mut st: &mut [Vf2State<Ty, Ix>; 2],
  }
  true
  };
  - let mut stack = vec![Frame::Outer];
  + let mut stack: Vec<Frame<NodeIndex<Ix>>> = vec![Frame::Outer];
  while let Some(frame) = stack.pop() {
  match frame {
  