I'm not sure why, but somehow `#[derive(Reflect)]` on a tuple struct
with a boxed trait object can result in linker errors when dynamic
linking is used on Windows using `rust-lld`:
```
= note: rust-lld: error: <root>: undefined symbol: bevy_animation::_::_$LT$impl$u20$bevy_reflect..reflect..PartialReflect$u20$for$u20$bevy_animation..AnimationEventFn$GT$::reflect_partial_eq::hc4cce1dc55e42e0b␍
rust-lld: error: <root>: undefined symbol: bevy_animation::_::_$LT$impl$u20$bevy_reflect..from_reflect..FromReflect$u20$for$u20$bevy_animation..AnimationEventFn$GT$::from_reflect::hc2b1d575b8491092␍
rust-lld: error: <root>: undefined symbol: bevy_animation::_::_$LT$impl$u20$bevy_reflect..tuple_struct..TupleStruct$u20$for$u20$bevy_animation..AnimationEventFn$GT$::clone_dynamic::hab42a4edc8d6b5c2␍
rust-lld: error: <root>: undefined symbol: bevy_animation::_::_$LT$impl$u20$bevy_reflect..tuple_struct..TupleStruct$u20$for$u20$bevy_animation..AnimationEventFn$GT$::field::h729a3d6dd6a27a43␍
rust-lld: error: <root>: undefined symbol: bevy_animation::_::_$LT$impl$u20$bevy_reflect..tuple_struct..TupleStruct$u20$for$u20$bevy_animation..AnimationEventFn$GT$::field_mut::hde1c34846d77344b␍
rust-lld: error: <root>: undefined symbol: bevy_animation::_::_$LT$impl$u20$bevy_reflect..type_registry..GetTypeRegistration$u20$for$u20$bevy_animation..AnimationEventFn$GT$::get_type_registration::hb96eb543e403a132␍
rust-lld: error: <root>: undefined symbol: bevy_animation::_::_$LT$impl$u20$bevy_reflect..type_registry..GetTypeRegistration$u20$for$u20$bevy_animation..AnimationEventFn$GT$::register_type_dependencies::hcf1a4b69bcfea6ae␍
rust-lld: error: undefined symbol: bevy_animation::_::_$LT$impl$u20$bevy_reflect..reflect..PartialReflect$u20$for$u20$bevy_animation..AnimationEventFn$GT$::reflect_partial_eq::hc4cce1dc55e42e0b␍
```
etc.
Adding `#[reflect(opaque)]` to the `Reflect` derive fixes the problem,
and that's what this patch does. I think that adding
`#[reflect(opaque)]` is harmless, as there's little that reflection
allows with a boxed trait object anyhow.
6ff36898ae