[wip] better #[derive(IntoEngineData)]

[zachschuermann]

stacked on #830

from @scovich - explore possible create_one changes

I guess support for nested structs would require us to derive From for structs, which produces Scalar::Struct? But how do struct scalars interact with create_one that expects leaf values? I almost wonder if we should change create_one to just take a Scalar to extract leaf values from? It comes with a schema and everything, including nullability info, so no need to pass a separate schema.