Method map_or

include/sol/optional_implementation.hpp:948–950

		/// \group map_or
		template <class F, class U>
		U map_or(F&& f, U&& u) & {
			return has_value() ? detail::invoke(std::forward<F>(f), **this) : std::forward<U>(u);
		}

		/// \group map_or
		template <class F, class U>