message: Print call trace to stderr.
diff: @@ -16,7 +16,7 @@ impl FunctionMiddleware for CallTrace {
  match op {
  Event::Internal(InternalEvent::FunctionBegin(id)) => {
  sink.push(Event::Internal(InternalEvent::Breakpoint(Box::new(move |_| {
  - println!("func ({})", id);
  + eprintln!("func ({})", id);
  }))))
  }
  _ => {}
  