Cast IMPs to the correct signature before calling

[HendrikHuebner]

This PR ensures we do not call any raw IMP function pointers without casting them to the correct type first.

Calling a funtion pointer with a mismatching signature is technically UB I believe but tends to work just fine on many native platforms.
With WebAssembly, this is a problem however, because WASM indirect calls require an exact signature match. This can cause runtime failures.

[davidchisnall]

How about something like this:

Suggested change

	typedef id (*NewAutoreleasePoolIMP)(id, SEL);
	typedef void (*DeleteAutoreleasePoolIMP)(id, SEL);
	typedef void (*AutoreleaseAddIMP)(id, SEL, id);
	static NewAutoreleasePoolIMP NewAutoreleasePool;
	static DeleteAutoreleasePoolIMP DeleteAutoreleasePool;
	static AutoreleaseAddIMP AutoreleaseAdd;
	template<typename Return, typename... Arguments>
	using Selector = Return(*)(id, SEL, Arguments...);
	static Selector<id> NewAutoreleasePool;
	static Selector<void> DeleteAutoreleasePoolIMP DeleteAutoreleasePool;
	static Selector<void, id> AutoreleaseAdd;

[davidchisnall]

Can we consolidate the casts with something like:

auto storeSelector = [](auto &target, IMP imp)
    {
         target = reinterpret_cast<std::remove_reference_t<decltype(target)>>(imp);
    };

[davidchisnall]

We should probably change the type of these from IMP and do the cast when we set them.