Function emplace

include/tabulate/optional_lite.hpp:672–674

  void construct_value(value_type &&v) { ::new (value_ptr()) value_type(std::move(v)); }

  template <class... Args> void emplace(Args &&... args) {
    ::new (value_ptr()) value_type(std::forward<Args>(args)...);
  }

  template <class U, class... Args> void emplace(std::initializer_list<U> il, Args &&... args) {
    ::new (value_ptr()) value_type(il, std::forward<Args>(args)...);