Function swap

src/include/cpp-btree/btree.h:193–195

template <class K, class V>
void swap(map_slot_type<K, V>& lhs, map_slot_type<K, V>& rhs) {
  std::swap(lhs.mutable_value, rhs.mutable_value);
}

// A parameters structure for holding the type parameters for a btree_map.
// Compare and Alloc should be nothrow copy-constructible.