The flecs cpp API invokes ub in flecs::opaque

[Gaspard--]

Flecs is invoking undefined behaviour by casting function pointers with reinterpret_cast in many functions of flecs::opaque and then calling them with the wrong signature:

flecs/include/flecs/addons/cpp/mixins/meta/opaque.hpp

Lines 46 to 51 in de8b378

	opaque& serialize(flecs::serialize<T> func) {
	this->desc.type.serialize =
	reinterpret_cast<decltype(
	this->desc.type.serialize)>(func);
	return *this;
	}

Caught using -fsanitize=undefined under clang.
The fix seems more complicated than #1416 because we need to somehow pass a function pointer that would call the other function pointer, or find some other trick.