Function combine_at_timestamp

pub fn combine_at_timestamp<G: Scope, Tr>(
    arranged: Arranged<G, Tr>,
) -> Collection<G, (Tr::KeyOwn, Vec<DiffPair<Tr::ValOwn>>), Diff>
where
    G::Timestamp: Lattice + Copy,
    Tr: Clone + TraceReader<Diff = Diff, Time = G::Timestamp, KeyOwn: Clone + 'static, ValOwn: 'static>,

Given a stream of batches, produce a stream of groups of DiffPairs, grouped by key, at each timestamp.