build: fix multiple Clang13 warnings

* spot/kripke/kripkegraph.hh, spot/misc/hash.hh, spot/twa/taatgba.cc,
  spot/twa/twagraph.hh, tests/core/ngraph.cc: Replace subtraction of
  pointeur minus nullptr by an explicit cast to size_t.
* spot/twa/acc.hh: Add explicit default copy assignment operator for
  rs_pair.

spot/misc/hash.hh

@@ -48,8 +48,7 @@ namespace spot
 
     size_t operator()(const T* p) const noexcept
     {
-      return knuth32_hash(reinterpret_cast<const char*>(p)
-                          - static_cast<const char*>(nullptr));
+      return knuth32_hash(reinterpret_cast<size_t>(p));
     }
   };