Method mutable_key_comp

src/include/cpp-btree/btree.h:1272–1274

  const node_type *root() const { return root_.data; }
  node_type *&mutable_root() { return root_.data; }
  key_compare *mutable_key_comp() noexcept {
    return static_cast<key_compare*>(&root_);
  }

  node_type* rightmost() {
    return rightmost_;