NFC: pointer_offset* functions always return void*

This requires that the caller perform the cast on the output rather than the
input, which is a little closer to the truth.  Shuffle some casts into the right
position.

src/ds/cdllist.h

@@ -31,7 +31,7 @@ namespace snmalloc
 
     SNMALLOC_FAST_PATH T* get_next()
     {
-      return pointer_offset_signed(static_cast<T*>(this), to_next);
+      return pointer_offset_signed<T>(this, to_next);
     }
   };