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/mem/superslab.h

@@ -183,8 +183,8 @@ namespace snmalloc
     Slab* alloc_slab(sizeclass_t sizeclass)
     {
       uint8_t h = head;
-      Slab* slab = pointer_offset(
-        reinterpret_cast<Slab*>(this), (static_cast<size_t>(h) << SLAB_BITS));
+      Slab* slab = reinterpret_cast<Slab*>(
+        pointer_offset(this, (static_cast<size_t>(h) << SLAB_BITS)));
 
       uint8_t n = meta[h].next;