ds/address::is_aligned_block: don't cast to size_t

Now that address_t is always a scalar, there's no reason to cast to
size_t.

src/ds/address.h

@@ -51,8 +51,7 @@ namespace snmalloc
   {
     static_assert(bits::next_pow2_const(alignment) == alignment);
 
-    return ((static_cast<size_t>(address_cast(p)) | size) & (alignment - 1)) ==
-      0;
+    return ((address_cast(p) | size) & (alignment - 1)) == 0;
   }
 
   /**