Better resource handling in minimization.

* src/tgbatest/ltl2tgba.cc (main): Delete the minimized automaton.
* src/tgbaalgos/minimize.cc (minimize): Remove the call to
unregister_variable() at the end.  It was both
wrong (unregistering only the first variable) and useless ("delete
del_a" will unregister all these variables).  Use a map and a set
to keep track of free BDD variable and reuse them, otherwise the
algorithm would sometimes use more variables than allocated.

src/tgbatest/ltl2tgba.cc

@@ -1247,6 +1247,7 @@ main(int argc, char** argv)
       delete system;
       delete expl;
       delete aut_red;
+      delete minimized;
       delete degeneralized;
       delete aut_scc;
       delete state_labeled;