Add support for coercing bevy mutable references to values

[MalekiRe]

[udoprog]

I fiddled a bit with the patch, so please take it for a spin. Instead of pulling in UnsafeToValue2 and calling unsafe_to_value you'll have to switch to using rune::bevy_support::anyobj_from_bevy_mut.

Some notes:

I like keeping everything in bevy_related. I also noted that AnyObjVTable is supposed to be public (but wasn't). So I've made it public.

AFAICT, using a trait UnsafeToValue2 doesn't net you much. You just have to pull it into scope anyways. So I've opted to just provide a function.

The reason we can't implement UnsafeToValue (which would be nice) is because of the ToValue blanket implementation. I'd definitely consider the blanket implementation because it would enable this which makes calling into rune better for you. This shouldn't be a big deal, since Any is mostly derived so we can easily just implement this as appropriate. Once this is implementable it can essentially just use the function.

MutUntyped has lifetimes, so MutUntyped is the same as MutUntyped<'static>. Coercing to 'static is a bit sketchy, but without bevy exposing raw APIs (which I think they should to enable sound uses like this) we don't have much of a choice.