message: Add Trace implementations for `fn`
  Summary: Not harmful, useful for something I'm working on, but even if that doesn't end up requiring them, still not a bad thing to have.
diff: @@ -239,3 +239,15 @@ unsafe impl<'v, T: Trace<'v>> Trace<'v> for Arc<Mutex<T>> {
  self.lock().unwrap().trace(tracer);
  }
  }
  +
  +unsafe impl<'v, A, R> Trace<'v> for fn(A) -> R {
  + fn trace(&mut self, _tracer: &Tracer<'v>) {}
  +}
  +
  +unsafe impl<'v, A, B, R> Trace<'v> for fn(A, B) -> R {
  + fn trace(&mut self, _tracer: &Tracer<'v>) {}
  +}
  +
  +unsafe impl<'v, A, B, C, R> Trace<'v> for fn(A, B, C) -> R {
  + fn trace(&mut self, _tracer: &Tracer<'v>) {}
  +}
  