Eliminate pointer_cast()

Since we anticipate address_t not carrying provenance on CHERI, but
rather being vaddr_t there, it doesn't make sense to offer conversion
back to a provenance-carrying pointer.

Thankfully, there is not much to be done here: the uses were few and
could be replaced with the vocabulary of other pointer operations in
ds/address.h

src/mem/superslab.h

@@ -181,8 +181,8 @@ namespace snmalloc
     Slab* alloc_slab(sizeclass_t sizeclass)
     {
       uint8_t h = head;
-      Slab* slab = pointer_cast<Slab>(
-        address_cast(this) + (static_cast<size_t>(h) << SLAB_BITS));
+      Slab* slab = pointer_offset(
+        reinterpret_cast<Slab*>(this), (static_cast<size_t>(h) << SLAB_BITS));
 
       uint8_t n = meta[h].next;