Replaced EntityCommand Implementation for FnOnce

[bushrat011899]

Objective

• Fixes EntityCommands get mutable reference to added components #4917
• Replaces Added Convenience Method For Entity Mutation #9602

Solution

• Replaced EntityCommand implementation for FnOnce to apply to FnOnce(EntityMut) instead of FnOnce(Entity, &mut World)

---

Changelog

• FnOnce(Entity, &mut World) no longer implements EntityCommand. This is a breaking change.

Migration Guide

1. New-Type FnOnce

Create an EntityCommand type which implements the method you previously wrote:

pub struct ClassicEntityCommand<F>(pub F);

impl<F> EntityCommand for ClassicEntityCommand<F>
where
    F: FnOnce(Entity, &mut World) + Send + 'static,
{
    fn apply(self, id: Entity, world: &mut World) {
        (self.0)(id, world);
    }
}

commands.add(ClassicEntityCommand(|id: Entity, world: &mut World| {
    /* ... */
}));

2. Extract (Entity, &mut World) from EntityMut

The method into_world_mut can be used to gain access to the World from an EntityMut.

let old = |id: Entity, world: &mut World| {
    /* ... */
};

let new = |mut entity: EntityMut| {
    let id = entity.id();
    let world = entity.into_world_mut();
    /* ... */
};

[alice-i-cecile]

Round three! I definitely like this as a final solution best of all :)

[bushrat011899]

Round three! I definitely like this as a final solution best of all :)

Fingers crossed! 😂

[JoJoJet]

The unsafe method world_mut can be used to gain access to the World from an EntityMut if a more reasonable refactor is not possible:

We should encourage people to use into_world_mut or world_scope instead. Otherwise, LGTM.

[bushrat011899]

We should encourage people to use into_world_mut or world_scope instead. Otherwise, LGTM.

Didn't know about that method! I've updated the Migration Guide for this PR to reflect the better alternative.