Switch todo!() to unreachable!/unimplemented! where it matters

examples/memo_chain.rs

@@ -8,7 +8,6 @@ fn app() -> Element {
     let mut value = use_signal(|| 0);
     let mut depth = use_signal(|| 0_usize);
     let items = use_memo(move || (0..depth()).map(|f| f as _).collect::<Vec<isize>>());
-
     let state = use_memo(move || value() + 1);
 
     println!("rendering app");