fn values_for_key(&self, index: usize) -> (usize, usize) {
            let lower = self.keys_offs.index(index);
            let upper = self.keys_offs.index(index+1);
            // Looking up values for an invalid key indicates something is wrong.
            assert!(lower < upper, "{:?} v {:?} at {:?}", lower, upper, index);
            (lower, upper)
        }
Lower and upper bounds in `self.vals` corresponding to the key at `index`.
differential-dataflow/src/trace/implementations/rhh.rs