run macro

[dakom]

This is already on @leudz Ali Baba's Cave (todo list) - just opening an issue for convenience

As discussed on Gitter...

The idea is to have a run macro that is similar in syntax to the system derive macro. The only major difference is that the world must be given as the first argument - and that it executes world.run() under the hood.

This would be valid syntax:

//create an entity
let entity = run!(&world, |mut entities:&mut Entities, mut labels:&mut Label| {
    entities.add_entity(&mut labels, Label::new("foo"))
});

//delete an entity
run!(&world, |mut all_storages: AllStorages| {
    all_storages.delete(entity);
});