Modify derive_label to support no_std environments (#15465)

# Objective

- Contributes to #15460

## Solution

- Wrap `derive_label` `quote!` in an anonymous constant which contains
an `extern crate alloc` statement, allowing use of the `alloc` namespace
even when a user has not brought in the crate themselves.

## Testing

- CI passed locally.

## Notes

We can't generate code that uses `::std::boxed::Box` in `no_std`
environments, but we also can't rely on `::alloc::boxed::Box` either,
since the user might not have declared `extern crate alloc`. To resolve
this, the generated code is wrapped in an anonymous constant which
contains the `extern crate alloc` invocation.

This does mean the macro is no longer hygienic against cases where the
user provides an alternate `alloc` crate, however I believe this is an
acceptable compromise.

Additionally, this crate itself doesn't need to be `no_std`, it just
needs to _generate_ `no_std` compatible code.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_macro_utils/src/label.rs

@@ -81,9 +81,13 @@ pub fn derive_label(
         .unwrap(),
     );
     quote! {
+        // To ensure alloc is available, but also prevent its name from clashing, we place the implementation inside an anonymous constant
+        const _: () = {
+            extern crate alloc;
+
             impl #impl_generics #trait_path for #ident #ty_generics #where_clause {
-            fn dyn_clone(&self) -> ::std::boxed::Box<dyn #trait_path> {
-                ::std::boxed::Box::new(::core::clone::Clone::clone(self))
+                fn dyn_clone(&self) -> alloc::boxed::Box<dyn #trait_path> {
+                    alloc::boxed::Box::new(::core::clone::Clone::clone(self))
                 }
 
                 fn as_dyn_eq(&self) -> &dyn #dyn_eq_path {
@@ -96,6 +100,7 @@ pub fn derive_label(
                     ::core::hash::Hash::hash(self, &mut state);
                 }
             }
+        };
     }
     .into()
 }