Macro facade 

macro_rules! facade {
    ($(#[$meta:meta])* $name:ident => $accessor:ty) => { ... };
}

Declare a facade type and its Facade impl.

pub struct Clock {
    pub now: u64,
}

facade!(
    /// Reads the application clock.
    Time => Clock
);

let app = Application::new(".");
app.instance(Clock { now: 1 });
rainier_container::set_facade_application(Arc::new(app));

assert_eq!(Time::instance().now, 1);