System Setup

[leudz]

This function would be triggered the first time a system is added to the scheduler.

It'll be a second method on the System trait alongside a second type SetupData.
The trait would become:

trait System<'a> {
    type Data: SystemData<'a>;
    type SetupData: SystemData<'a>;
    fn run(storage: <Self::Data as SystemData<'a>>::View);
    fn setup(_: <Self::SetupData as SystemData<'a>>::View) {}
}

Default associated types are not stable yet so everyone will have to define SetupData, fortunately there'll be a macro:

#[system_with_setup]
impl InAcid {
    fn run(/*borrows*/) { ... }
    fn setup(/*setup borrows*/) { ... }
}