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/kripke/kripkegraph.hh

@@ -64,8 +64,7 @@ namespace spot
 
     virtual size_t hash() const override
     {
-      return
-        reinterpret_cast<const char*>(this) - static_cast<const char*>(nullptr);
+      return reinterpret_cast<size_t>(this);
     }
 
     virtual kripke_graph_state*

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));
     }
   };
 

spot/twa/acc.hh

@@ -1670,6 +1670,7 @@ namespace spot
 #ifndef SWIG
       rs_pair() = default;
       rs_pair(const rs_pair&) = default;
+      rs_pair& operator=(const rs_pair&) = default;
 #endif
 
       rs_pair(acc_cond::mark_t fin, acc_cond::mark_t inf) noexcept:

spot/twa/taatgba.cc

@@ -105,7 +105,7 @@ namespace spot
     taa_tgba::state_set::const_iterator it = s_->begin();
     while (it != s_->end())
     {
-      res ^= reinterpret_cast<const char*>(*it++) - static_cast<char*>(nullptr);
+      res ^= reinterpret_cast<size_t>(*it++);
       res = wang32_hash(res);
     }
     return res;

spot/twa/twagraph.hh

@@ -70,8 +70,7 @@ namespace spot
 
     virtual size_t hash() const override
     {
-      return
-        reinterpret_cast<const char*>(this) - static_cast<const char*>(nullptr);
+      return reinterpret_cast<size_t>(this);
     }
 
     virtual twa_graph_state*

tests/core/ngraph.cc

@@ -372,8 +372,7 @@ public:
 
   size_t hash() const override
   {
-    return
-      reinterpret_cast<const char*>(this) - static_cast<const char*>(nullptr);
+    return reinterpret_cast<size_t>(this);
   }
 
   my_state* clone() const override