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

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

src/pal/pal_consts.h

@@ -78,7 +78,7 @@ namespace snmalloc
     /**
      * List of callbacks to notify
      */
-    std::atomic<PalNotificationObject*> callbacks = nullptr;
+    std::atomic<PalNotificationObject*> callbacks{nullptr};
 
   public:
     /**