Add `Self: Sized` bounds to `Bundle` trait methods

[jnhyatt]

Objective

Fixes https://github.com/bevyengine/bevy/issues/3227.

I'll admit upfront this might be some silly shenanigans.

This came about based on some Rust-fu I was trying to do to enable generic dynamic bundle insertion/removal. Consider the following snippet:

#[derive(Component)]
pub struct InsertThisLater(Box<dyn Bundle>);

This doesn't compile, and from my perspective, understandably: a Bundle that doesn't know its type can't get into the world. A workaround I came up with is this:

pub trait DynBundle {
    fn insert(self: Box<Self>, mut e: EntityWorldMut);
}

pub struct InsertThisLater(Box<DynBundle>);

A component can impl this trait (or derive it) and suddenly you have a working system! Only problem is Rust thinks it's not possible since some of Bundle's methods (erroneously) assume Self: ?Sized.

Solution

Add where Self: Sized to the rest of Bundle's (and DynamicBundle's) trait methods

---

Breaking change?

Technically, I think?

I'm not exactly sure what kind of code you'd have to write to run into this breaking change, but this change expands what you can do with bundles, not the other way around. I'll try and find some weird bit of code that breaks with this, but I don't think it needs a migration guide section, and honestly, I don't think it would be bad to shove it into 0.14.2, however I understand that still breaks the letter of the law and is a solid argument against doing so.