Prevent use-after-free on scope-stored data

[DasLixou]

Things like

cx.sub_scope(|cx: &mut Context<'_, '_>| {
    let val = cx.store(42);
    cx.close_scope(cx.scope);
    println!("{val}");
});

shouldn't be possible.
This should be a compile time error via lifetimes somehow.
Also this could be a problem:

cx.sub_scope(|cx: &mut Context<'_, '_>| {
    let val = cx.store(42);
    let kill_switch = EventStation::new();
    let s = cx.scope;
    station.listen(
        s,
        move |_event: &KillSignal, cx: &mut Context<'_, '_>| {
            cx.close_scope(s);
        },
    );
    // this will be a problem:
    kill_switch.fire();
    println!("{val}");
    // while this would be fine:
    let button = /* .. */;
    button.on_press.listen(s, |_, _| { kill_switch.fire(); });
});

[DasLixou]

One idea that is not directly related to that but could help: Storing all stores in one big store and then having it indexed by a storekey with lifetime maybe?

[DasLixou]

Ideally we don't even return to the building function or store the close_scopes up till all building is done? (but that may be a little bit of waiting when theres lots to build)