ds/address: add pointer diff function

And use it rather than open-coding subtraction of two address_cast-s.

src/mem/slab.h

@@ -16,7 +16,7 @@ namespace snmalloc
     uint16_t pointer_to_index(void* p)
     {
       // Get the offset from the slab for a memory location.
-      return static_cast<uint16_t>(address_cast(p) - address_cast(this));
+      return static_cast<uint16_t>(pointer_diff(this, p));
     }
 
   public:
@@ -144,7 +144,7 @@ namespace snmalloc
       Metaslab& meta = super->get_meta(this);
       return is_multiple_of_sizeclass(
         sizeclass_to_size(meta.sizeclass),
-        address_cast(this) + SLAB_SIZE - address_cast(p));
+        pointer_diff(p, pointer_offset(this, SLAB_SIZE)));
     }
 
     // Returns true, if it deallocation can proceed without changing any status