Optimizing closure and sl.

* src/tgbaalgos/closure.cc, src/tgbaalgos/closure.hh: Using vectors
instead of sets and unordered maps, adding an overload to handle rvalue
references.
* src/tgbaalgos/stutterize.cc, src/tgbaalgos/stutterize.hh: Adding
an overload to handle rvalue references.
* bench/stutter/stutter_invariance_formulas.cc,
bench/stutter/stutter_invariance_randomgraph.cc: Automata are modified
in-place by is_stutter_invariant so they have to be copied before being
processed.
* src/tgbaalgos/stutter_invariance.cc,
src/tgbaalgos/stutter_invariance.hh: Use the in-place version of
closure and sl.

src/tgbaalgos/stutterize.hh

@@ -38,6 +38,11 @@ namespace spot
 
   SPOT_API tgba_digraph_ptr
   sl2(const const_tgba_digraph_ptr&, bdd);
+
+#ifndef SWIG
+  SPOT_API tgba_digraph_ptr
+  sl2(tgba_digraph_ptr&&);
+#endif
 }
 
 #endif