genltl: move all formula generation code to spot/gen/

Fixes #254.

* spot/gen/formulas.cc, spot/gen/formulas.hh: New files.
* spot/gen/Makefile.am: Add them.
* spot/Makefile.am: Fix build order.
* bin/genltl.cc: Move most code to the above files and adjust.
* bin/Makefile.am: Link genltl with libspotgen.
* doc/org/arch.tex: Adjust picture to show that genltl uses
libspotgen.
* python/spot/gen.i: Include formulas.hh.
* tests/python/gen.py: Make sure genltl() and ltl_pattern_name()
can be called.

bin/Makefile.am

@@ -1,6 +1,6 @@
 ## -*- coding: utf-8 -*-
-## Copyright (C) 2012, 2013, 2014, 2015, 2016-2017 Laboratoire de Recherche
-## et Développement de l'Epita (LRDE).
+## Copyright (C) 2012-2017 Laboratoire de Recherche et Développement
+## de l'Epita (LRDE).
 ##
 ## This file is part of Spot, a model checking library.
 ##
@@ -79,6 +79,7 @@ ltlfilt_SOURCES = ltlfilt.cc
 genaut_SOURCES = genaut.cc
 genaut_LDADD = $(top_builddir)/spot/gen/libspotgen.la $(LDADD)
 genltl_SOURCES = genltl.cc
+genltl_LDADD = $(top_builddir)/spot/gen/libspotgen.la $(LDADD)
 randaut_SOURCES = randaut.cc
 randltl_SOURCES = randltl.cc
 ltl2tgba_SOURCES = ltl2tgba.cc