Add tuple application utility and size retrieval in fast_io_dsal

- Introduced `apply` function to facilitate the application of a callable to elements of a tuple.
- Added `tuple_size` function to retrieve the size of a `fast_io::containers::tuple`.
- Implemented helper functions in the `details` namespace for improved code organization and clarity.

Author: MacroModel

include/fast_io_dsal/impl/tuple.h

@@ -182,6 +182,32 @@ FAST_IO_GNU_ALWAYS_INLINE
 		static_cast<decltype(::fast_io::containers::details::get_tuple_element_by_type_<T, 0, Args...>())::type const &&>(self).val_);
 }
 
+namespace details
+{
+template <typename F, typename Tuple, std::size_t... I>
+inline constexpr decltype(auto) apply_impl(F &&f, Tuple &&t, ::std::index_sequence<I...>)
+{
+	return ::std::forward<F>(f)(get<I>(::std::forward<Tuple>(t))...);
+}
+
+template <typename... Args>
+inline consteval ::std::size_t tuple_size(::fast_io::containers::tuple<Args...> const &) noexcept
+{
+	return sizeof...(Args);
+}
+
+} // namespace details
+
+template <typename F, typename Tuple>
+inline constexpr decltype(auto) apply(F &&f, Tuple &&t)
+{
+	constexpr ::std::size_t N{details::tuple_size(t)};
+	return details::apply_impl(
+		::std::forward<F>(f),
+		::std::forward<Tuple>(t),
+		::std::make_index_sequence<N>{});
+}
+
 namespace details
 {