mpark's excellent variant implementation is conditionally used by us if we type stdx::variant and <variant> is not present. Since this is never the case on any current compiler, we are always using an inferior implementation. Furthermore std::visit is not implemented by one of our current compilers, leading to <variant> being incomplete but chosen over mpark anyway.

We should remove the ifdef conditions on mpark::variant so it is always chosen as the implementation of stdx::variant. The standard library version is now always available through std:: anyway.