ngraph: support aliasing states

* src/graph/ngraph.hh (alias_state): New method.
* src/graphtest/ngraph.cc: Use it.

src/graph/ngraph.hh

@@ -77,6 +77,13 @@ namespace spot
       return p.first->second;
     }
 
+    /// \brief Give an alternate name to a state.
+    /// \return true iff the newname was already used.
+    bool alias_state(state s, name newname)
+    {
+      return !name_to_state.insert(std::make_pair(newname, s)).second;
+    }
+
     state get_state(name n) const
     {
       return name_to_state.at(n);

src/graphtest/ngraph.cc

@@ -389,10 +389,12 @@ bool f9()
   spot::named_graph<graph_t, std::string> gg(g);
   auto s1 = gg.new_state("s1");
   gg.new_state("s2");
-  gg.new_state("s3");
+  auto s3 = gg.new_state("s3");
+  gg.alias_state(s3, "s3b");
+
   gg.new_transition("s1", "s2", 1, 3);
   gg.new_transition("s1", "s3", 2, 5);
-  gg.new_transition("s2", "s3", 3, 7);
+  gg.new_transition("s2", "s3b", 3, 7);
   gg.new_transition("s3", "s2", 4, 9);
 
   dot(std::cout, gg);