tgbagraph: add a merge_transitions() method.

* src/graph/graph.hh: Add some framework to erase transitions, and
defrag the resulting transitions_ vector on demand.  Also remove
the nb_states() and nb_transitions() because num_states() and
num_transitions() already exist.
* src/graphtest/graph.cc, src/graphtest/ngraph.cc: Adjust to
use num_states().
* src/tgba/tgbagraph.hh (merge_transitions): New method.
* src/misc/hash.hh: Add a pair_hash class, needed by
merge_transitions().
* src/graphtest/tgbagraph.cc, src/graphtest/tgbagraph.test: Add states
for transitions removal and merge_transitions().

src/misc/hash.hh

@@ -75,6 +75,18 @@ namespace spot
       return s;
     }
   };
+
+
+  struct pair_hash
+  {
+    template<typename T, typename U>
+    std::size_t operator()(const std::pair<T, U> &p) const
+    {
+      return wang32_hash(static_cast<size_t>(p.first) ^
+			 static_cast<size_t>(p.second));
+    }
+  };
+
 }
 
 #endif // SPOT_MISC_HASH_HH