Macro task 

macro_rules! task {
    ($pool_type:path, $block:expr) => { ... };
    ($pool_type:path, $event:expr, $block:expr) => { ... };
    (@inner $pool_type:path, $block:expr $(, $event:expr)?) => { ... };
}

Creates an asychronously executing task. When finished, may optionally send an event.

Example usage:

let task = task!(
    IoTaskPool,
    MyEvent::default(), // (optional)
    async move |q: &mut CommandQueue| {
        // do some async stuff
        q.push(|world: &mut World| {
            // do some world mutation
        });
    });
task(world);