fn copy_key(&mut self, source: &OrdKeyStorage<L>, cursor: usize) {
            self.stash_updates_for_key(source, cursor);
            if let Some(off) = self.consolidate_updates() {
                self.result.keys_offs.push(off);
                self.result.keys.push(source.keys.index(cursor));
            }
        }
Copy the next key in `source`.

The method extracts the key in `source` at `cursor`, and merges it in to `self`.
If the result does not wholly cancel, they key will be present in `self` with the
compacted values and updates.

The caller should be certain to update the cursor, as this method does not do this.
differential-dataflow/src/trace/implementations/ord_neu.rs