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.cc

@@ -518,6 +518,8 @@ hoa_stat_printer::print(const spot::const_parsed_aut_ptr& haut,
   output_aut_ = nullptr;
   input_aut_ = nullptr;
   haut_scc_.reset();
+  aut_ap_.clear();
+  haut_ap_.clear();
   return res;
 }
 

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();
   }

bin/common_output.cc

@@ -239,7 +239,13 @@ namespace
         size_ = spot::length(f);
       if (has('h'))
         class_ = spot::mp_class(f);
-      return format(format_);
+      auto& res = format(format_);
+      // Make sure we do not store the formula until the next one is
+      // printed, as the order in which APs are registered may
+      // influence the automata output.
+      fl_ = nullptr;
+      ap_.clear();
+      return res;
     }
 
   private: