message: Add type alias for get_cell_label
diff: @@ -130,11 +130,14 @@ std::ostream& operator<<(std::ostream& stream, const AlignmentCell& cell) {
  return stream;
  }
  +template <typename ValueType>
  +using CellLabelGetterFunc =
  + std::function<std::pair<std::string, char>(const VectorTree<ValueType>&)>;
  +
  template <typename ValueType, typename Cell>
  static std::size_t CreateTextNodes(
  const VectorTree<ValueType>& src_node, VectorTree<Cell>* dst_node,
  - const std::function<std::pair<std::string, char>(
  - const VectorTree<ValueType>&)>& get_cell_label) {
  + const CellLabelGetterFunc<ValueType>& get_cell_label) {
  static constexpr std::size_t kMinCellWidth = 2;
  std::size_t depth = 0;
  @@ -158,8 +161,7 @@ static std::size_t CreateTextNodes(
  template <typename ValueType>
  static void ColumnsTreeFormatter(
  std::ostream& stream, const VectorTree<ValueType>& root,
  - const std::function<std::pair<std::string, char>(
  - const VectorTree<ValueType>&)>& get_cell_label) {
  + const CellLabelGetterFunc<ValueType>& get_cell_label) {
  if (root.Children().empty()) return;
  static constexpr absl::string_view kCellSeparator = "|";
  