Automatic registration example for platforms without inventory support

This example illustrates how to use automatic type registration of bevy_reflect on platforms that don't support inventory.

To run the example, use the provided Makefile with make run or run manually by setting env var and enabling the required feature:

BEVY_REFLECT_AUTO_REGISTER_STATIC=1 cargo run --features bevy/reflect_auto_register_static

This approach should generally work on all platforms, however it is less convenient and slows down linking. It's recommended to use it only as a fallback.

Here's a list of caveats of this approach:

1. load_type_registrations! macro must be called before constructing App or using TypeRegistry::register_derived_types.
2. All of the types to be automatically registered must be declared in a separate from load_type_registrations! crate. This is why this example uses separate lib and bin setup.
3. Registration function names are cached in target/type_registrations. Due to incremental compilation the only way to rebuild this cache is to build with bevy/reflect_auto_register_static (or auto_register_static if just using bevy_reflect) feature disabled, then delete target/type_registrations and rebuild again with this feature enabled and BEVY_REFLECT_AUTO_REGISTER_STATIC=1 environment variable set. Running cargo clean before recompiling is also an option, but it is even slower to do.

If you're experiencing linking issues try running cargo clean before rebuilding.