XRBK macro doesn't add generic bounds

[Antikyth]

xrbk_macro should add bounds to generics in a type when implementing XRBK traits for it. For example, every element that doesn't have a #[hide] attribute would have to implement X11Size before X11Size could be derived for the struct or enum.

[Antikyth]

See the HeapSize example in syn for inspiration perhaps, as it does this.