XOR encoded next_object

This commit adds a simple XOR encoding to the next_object pointer in
FreeObjects.  This removes the trivial way of getting hold of a physical
address from the system by observing the free list pointers in
deallocated objects.

src/ds/defines.h

@@ -130,4 +130,13 @@ check_client_impl(bool test, const char* const str)
 #  define check_client(test, str) check_client_impl(test, str)
 #else
 #  define check_client(test, str)
-#endif
+#endif
+
+namespace snmalloc
+{
+#ifdef SNMALLOC_CHECK_CLIENT
+  static constexpr bool CHECK_CLIENT = true;
+#else
+  static constexpr bool CHECK_CLIENT = false;
+#endif
+} // namespace snmalloc