bin: release all subformulas between runs

Fixes #262, reported by Maximilien Colange.

* bin/common_output.cc, bin/common_aoutput.cc, bin/common_aoutput.hh:
Clear the set of atomic propositions if --stats=%[...]x was used.
* spot/twa/bdddict.cc: Release any formula associated to a BDD when it
is unregistered, do not wait for the dictionary's destruction.  This
was the main culprit for #262.
* tests/core/ltl2tgba.test: Add test cases.
* NEWS: Mention the bug.

bin/common_aoutput.hh

@@ -121,11 +121,15 @@ protected:
               });
   }
 public:
+  void clear()
+  {
+    val_.clear();
+  }
 
   template<class T>
   void set(T begin, T end)
   {
-    val_.clear();
+    clear();
     val_.insert(val_.end(), begin, end);
     sort();
   }