Function each 

pub fn each<S, P>(
    source: S,
    build_row: impl Fn(S::Slot) -> P + 'static,
) -> impl Piece
where
    S: RowSource + 'static,
    P: Piece,

Reactive keyed collection (§5.4): keyed diff, per-key child scopes, slot rebinds for surviving rows, debug key-uniqueness assertion. Takes any RowSource.