Various minor changes to aid compiling with std14 (#182)

These changes make the code compile in clang10 with -std14.

src/ds/aba.h

@@ -121,6 +121,7 @@ namespace snmalloc
       }
 
       Cmp(const Cmp&) = delete;
+      Cmp(Cmp&&) noexcept = default;
     };
 
     // This method is used in Verona

src/ds/helpers.h

@@ -14,7 +14,7 @@ namespace snmalloc
   class Singleton
   {
     inline static std::atomic_flag flag;
-    inline static std::atomic<bool> initialised = false;
+    inline static std::atomic<bool> initialised{false};
     inline static Object obj;
 
   public:

src/ds/mpscq.h

@@ -14,7 +14,7 @@ namespace snmalloc
       std::is_same<decltype(T::next), std::atomic<T*>>::value,
       "T->next must be a std::atomic<T*>");
 
-    std::atomic<T*> back = nullptr;
+    std::atomic<T*> back{nullptr};
     T* front = nullptr;
 
   public:
@@ -72,10 +72,10 @@ namespace snmalloc
         SNMALLOC_ASSERT(front);
         std::atomic_thread_fence(std::memory_order_acquire);
         invariant();
-        return std::pair(first, true);
+        return {first, true};
       }
 
-      return std::pair(nullptr, false);
+      return {nullptr, false};
     }
   };
 } // namespace snmalloc