Fix boxed labels

[JoJoJet]

Objective

Label traits such as ScheduleLabel currently have a major footgun: the trait is implemented for Box<dyn ScheduleLabel>, but the implementation does not function as one would expect since Box<T> is considered to be a distinct type from T. This is because the behavior of the ScheduleLabel trait is specified mainly through blanket implementations, which prevents Box<dyn ScheduleLabel> from being properly special-cased.

Solution

Replace the blanket-implemented behavior with a series of methods defined on ScheduleLabel. This allows us to fully special-case Box<dyn ScheduleLabel> .

---

Changelog

Fixed a bug where boxed label types (such as Box<dyn ScheduleLabel>) behaved incorrectly when compared with concretely-typed labels.

Migration Guide

The ScheduleLabel trait has been refactored to no longer depend on the traits std::any::Any, bevy_utils::DynEq, and bevy_utils::DynHash. Any manual implementations will need to implement new trait methods in their stead.

impl ScheduleLabel for MyType {
    // Before:
    fn dyn_clone(&self) -> Box<dyn ScheduleLabel> { ... }

    // After:
    fn dyn_clone(&self) -> Box<dyn ScheduleLabel> { ... }

    fn as_dyn_eq(&self) -> &dyn DynEq {
        self
    }

    // No, `mut state: &mut` is not a typo.
    fn dyn_hash(&self, mut state: &mut dyn Hasher) {
        self.hash(&mut state);
        // Hashing the TypeId isn't strictly necessary, but it prevents collisions.
        TypeId::of::<Self>().hash(&mut state);
    }
}