Adding tgba-based stutter-invariance checking

* src/tgbaalgos/closure.cc, src/tgbaalgos/closure.hh:
Add closure function.
* src/tgbaalgos/stutterize.cc, src/tgbaalgos/stutterize.hh:
Add two implementations of "self-loopize" function.
* src/tgbaalgos/Makefile.am: Add them.
* src/tgba/tgbasl.cc, src/tgba/tgbasl.hh: On-the-fly implementation of
self-loopize.
* src/tgba/Makefile.am: Add it.
* src/tgbatest/ltl2tgba.cc, src/tgbatest/stutter_invariant.test: Test
closure and sl.
* src/tgbatest/Makefile.am: Adjust.
* src/bin/ltlfilt.cc: Modify stutter-invariant option to use
automaton-based checking rather than syntactic-based checking.
* src/ltlvisit/remove_x.cc, src/ltlvisit/remove_x.hh:
Remove is_stutter_insensitive function.
* src/tgbaalgos/stutter_invariance.cc,
src/tgbaalgos/stutter_invariance.hh: Check if a formula is
stutter-invariant using closure and sl.
* wrap/python/spot.i: Add closure and sl bindings.
* bench/stutter/stutter_invariance_formulas.cc: Generate benchmarks from
given formulas.
* bench/stutter/stutter_invariance_randomgraph.cc: Generate benchmarks
from random automata.
* bench/stutter/Makefile.am: Add them.
* configure.ac: Add bench/stutter/Makefile.
* bench/Makefile.am: Add stutter subdirectory.
* README: Document bench/stutter directory.

bench/stutter/Makefile.am

@@ -0,0 +1,31 @@
+## -*- coding: utf-8 -*-
+## Copyright (C) 2014 Laboratoire de Recherche et Développement
+## de l'Epita (LRDE).
+##
+## This file is part of Spot, a model checking library.
+##
+## Spot is free software; you can redistribute it and/or modify it
+## under the terms of the GNU General Public License as published by
+## the Free Software Foundation; either version 3 of the License, or
+## (at your option) any later version.
+##
+## Spot is distributed in the hope that it will be useful, but WITHOUT
+## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+## or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+## License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+SUBDIRS = .
+
+AM_CPPFLAGS = -I$(top_srcdir)/src -I$(top_builddir)/src $(BUDDY_CPPFLAGS) \
+              -I$(top_builddir)/lib -I$(top_srcdir)/lib
+AM_CXXFLAGS = $(WARNING_CXXFLAGS)
+LDADD = $(top_builddir)/src/bin/libcommon.a ../../lib/libgnu.la ../../src/libspot.la
+
+bin_PROGRAMS = stutter_invariance_randomgraph \
+	       stutter_invariance_formulas
+
+stutter_invariance_randomgraph_SOURCES = stutter_invariance_randomgraph.cc
+stutter_invariance_formulas_SOURCES = stutter_invariance_formulas.cc