Function build_halfjoin2 

fn build_halfjoin2<G, Tr, CF>(
    updates: VecCollection<G, (Row, Row, G::Timestamp), Diff>,
    trace: Arranged<G, Tr>,
    comparison: CF,
    closure: JoinClosure,
    datums: DatumVec,
    errs: VecCollection<G, DataflowError, Diff>,
) -> (VecCollection<G, (Row, G::Timestamp), Diff>, VecCollection<G, DataflowError, Diff>)
where
    G: Scope,
    G::Timestamp: RenderTimestamp,
    Tr: TraceReader<KeyOwn = Row, Time = G::Timestamp, Diff = Diff> + Clone + 'static,
    for<'a> Tr::Val<'a>: ToDatumIter,
    CF: Fn(Tr::TimeGat<'_>, &G::Timestamp) -> bool + 'static,

half_join2 implementation (less-quadratic, new default).