fix scene loading

[cart]

Fixes #986

We were largely just missing a bunch of #[reflect(Component)] calls, but also some type registrations. We now automatically register Handle<T> when an asset is registered.

I did need to make Handle<T>::from_weak_u64 non-const to make the derive work (I moved that method to HandleUntyped). I'm considering a manual Handle reflect impl instead (which would probably allow us to keep const Handle<T>

[cart]

I think I'm cool with using HandleUntyped for const handles instead of Handle<T>. We can always re-add const Handle<T> when #![feature(const_fn)] stabilizes.