Make signal callable on stable

[ealmloff]

(Thanks to @matthunz for letting me know about dtolnay's amazing article about derefing to closures on stable!)

This restores the previously unsound implementation of Deref<Target = dyn Fn() -> Ref<'static, T>> on signals which lets you call the signal like a function instead of reading the signal:

fn App(cx: Scope) -> Element {
    let signal = Signal::new("hello world".to_string());
    // You can call signals like functions to get a Ref of their value.
    assert_eq!(&*signal(), "hello world");

    render! {
        "hello world"
    }
}

[jkelleyrtp]

What's cool about this is you can now do very simple iters:

for todo in todos().iter() { 
    div { "{todo.title}" }
}

Looks good