Factored casts out of DLL/mpmcstack/mpscq.

Nicer use of decltype to constrain instances.

Made a terminator method, to factor out casts.

src/ds/mpmcstack.h

@@ -11,7 +11,7 @@ namespace snmalloc
 
   private:
     static_assert(
-      std::is_same<decltype(((T*)0)->next), std::atomic<T*>>::value,
+      std::is_same<decltype(T::next), std::atomic<T*>>::value,
       "T->next must be a std::atomic<T*>");
 
     ABA stack;