Method index_into_right_array

cpp/src/arrow/util/ree_util.h:514–514

  int64_t index_into_left_array() const { return index_into_array<0>(); }
  int64_t index_into_right_array() const { return index_into_array<1>(); }

  MergedRunsIterator& operator++() {
    auto& left_it = std::get<0>(ree_iterators_);