Fixed GNU compiler warnings

[jnyfah]

• returning with std::move has no effect as some is already a temporary object

STX/include/stx/option.h

Lines 666 to 670 in 822cf2a

	constexpr auto take() -> Option {
	if (is_some()) {
	Some some = std::move(some_);
	storage::assign(None);
	return std::move(some);

• using decltype((void)) serves the same purpose of indicating that the type T has an operator() member function.

STX/include/stx/fn.h

Lines 173 to 174 in 822cf2a

	struct is_functor_impl<T, decltype(&T::operator(), (void)0)>
	: public std::true_type {};

• No need to have an empty if block

STX/include/stx/vec.h

Lines 23 to 25 in 822cf2a

	if constexpr (std::is_trivially_destructible_v<T>) {
	} else {
	for (T& element : Span<T>{start, size}) {

[lamarrr]

LGTM. Thanks for your contribution! 😄