Function zip_into_diff_pairs 

fn zip_into_diff_pairs<'scope>(
    sink_id: GlobalId,
    sink: &StorageSinkDesc<CollectionMetadata, Timestamp>,
    sink_render: &dyn SinkRender<'scope>,
    collection: VecCollection<'scope, Timestamp, Row, Diff>,
) -> VecCollection<'scope, Timestamp, (Option<Row>, DiffPair<Row>), Diff>

Zip the input to a sink so that updates to the same key appear as DiffPairs.