by not calling `std::atomic<T>::atomic(void)`.
After the latter the instance "does not contain a T object, and its only valid uses are destruction and initialization by std::atomic_init" which we don't call. So the only safe option is `std::atomic<T>::atomic(T)`.
https://en.cppreference.com/w/cpp/atomic/atomic/atomic
Apparently there was a reason for making the members of generated classes
atomic. However, this was only done for some types, others were still accessed
using non-atomic operations. For members of type T::Ptr (i.e. intrusive_ptr<T>),
this can result in a double free when multiple threads access the same variable
and at least one of them writes to the variable.
This commit makes use of std::atomic<T> for more T (it removes the additional
constraint sizeof(T) <= sizeof(void*)) and uses a type including a mutex for
load and store operations as a fallback.
