Function each 

pub fn each<T, K, P>(
    items: impl Fn() -> Vec<T> + 'static,
    key_of: impl Fn(&T) -> K + 'static,
    build_row: impl Fn(ItemSlot<T, K>) -> P + 'static,
) -> AnyPiece
where
    T: Clone + 'static,
    K: Eq + Hash + Clone + 'static,
    P: Piece,

Reactive keyed collection (§5.4): keyed diff, per-key child scopes, slot writes for surviving keys, debug key-uniqueness assertion.