Resources with a lifetime

Currently `Resource`s must be `'static`, which makes it difficult to implement many functionalities as systems, notably rendering, since the textures / sprites / render contexts etc. usually have a lifetime associated with them.

My question is, is it possible to have resources that are generic over a lifetime parameter `'a` and have `Schedule::execute<'a>(&mut self, world: &mut World, resources: &'a mut Resource<'a>`? After all, resources are not accessed outside of this function. Or is there some reason against this?