Make LowMemoryNotification object allocated (#281)

* Make LowMemoryNotification object allocated

This makes a separate allocation for the callback object.  This makes
it easier for different callbacks to be used.

* Add reserve_with_leftover to address_space

The address_space now supports reserving for non-power of 2 allocations
and the space that is used for rounding up is retained by the
address_space.  This means that we can more tightly pack the allocators
internal objects.

src/pal/pal_consts.h

@@ -70,9 +70,13 @@ namespace snmalloc
    */
   struct PalNotificationObject
   {
-    std::atomic<PalNotificationObject*> pal_next;
+    std::atomic<PalNotificationObject*> pal_next = nullptr;
 
     void (*pal_notify)(PalNotificationObject* self);
+
+    PalNotificationObject(void (*pal_notify)(PalNotificationObject* self))
+    : pal_notify(pal_notify)
+    {}
   };
 
   /***