fn advance_key(&self, index: usize, key: <L::KeyContainer as BatchContainer>::ReadItem<'_>) -> bool {
            // Ideally this short-circuits, as `self.keys[index]` is bogus data.
            !self.live_key(index) || self.keys.index(index).lt(&<L::KeyContainer as BatchContainer>::reborrow(key))
        }
Returns true if one should advance one's index in the search for `key`.
differential-dataflow/src/trace/implementations/rhh.rs