kill the ltl namespace

* NEWS: Mention it.
* bench/stutter/stutter_invariance_formulas.cc,
bench/stutter/stutter_invariance_randomgraph.cc, doc/mainpage.dox,
doc/org/tut01.org, doc/org/tut02.org, doc/org/tut10.org, doc/tl/tl.tex,
iface/ltsmin/ltsmin.cc, iface/ltsmin/ltsmin.hh,
iface/ltsmin/modelcheck.cc, src/bin/autfilt.cc,
src/bin/common_aoutput.cc, src/bin/common_aoutput.hh,
src/bin/common_finput.cc, src/bin/common_finput.hh,
src/bin/common_output.cc, src/bin/common_output.hh, src/bin/common_r.hh,
src/bin/common_trans.cc, src/bin/common_trans.hh, src/bin/dstar2tgba.cc,
src/bin/genltl.cc, src/bin/ltl2tgba.cc, src/bin/ltl2tgta.cc,
src/bin/ltlcross.cc, src/bin/ltldo.cc, src/bin/ltlfilt.cc,
src/bin/ltlgrind.cc, src/bin/randaut.cc, src/bin/randltl.cc,
src/kripke/kripkeexplicit.cc, src/kripke/kripkeexplicit.hh,
src/kripkeparse/kripkeparse.yy, src/kripkeparse/public.hh,
src/ltlparse/fmterror.cc, src/ltlparse/ltlparse.yy,
src/ltlparse/ltlscan.ll, src/ltlparse/parsedecl.hh,
src/ltlparse/public.hh, src/parseaut/parseaut.yy,
src/parseaut/public.hh, src/tests/checkpsl.cc, src/tests/checkta.cc,
src/tests/complementation.cc, src/tests/consterm.cc,
src/tests/emptchk.cc, src/tests/equalsf.cc, src/tests/ikwiad.cc,
src/tests/kind.cc, src/tests/length.cc, src/tests/ltlprod.cc,
src/tests/ltlrel.cc, src/tests/parse.test,
src/tests/parse_print_test.cc, src/tests/randtgba.cc,
src/tests/readltl.cc, src/tests/reduc.cc, src/tests/syntimpl.cc,
src/tests/taatgba.cc, src/tests/tostring.cc, src/tests/tostring.test,
src/tl/apcollect.cc, src/tl/apcollect.hh, src/tl/contain.cc,
src/tl/contain.hh, src/tl/declenv.cc, src/tl/declenv.hh,
src/tl/defaultenv.cc, src/tl/defaultenv.hh, src/tl/dot.cc,
src/tl/dot.hh, src/tl/environment.hh, src/tl/exclusive.cc,
src/tl/exclusive.hh, src/tl/formula.cc, src/tl/formula.hh,
src/tl/length.cc, src/tl/length.hh, src/tl/mark.cc, src/tl/mark.hh,
src/tl/mutation.cc, src/tl/mutation.hh, src/tl/nenoform.cc,
src/tl/nenoform.hh, src/tl/print.cc, src/tl/print.hh,
src/tl/randomltl.cc, src/tl/randomltl.hh, src/tl/relabel.cc,
src/tl/relabel.hh, src/tl/remove_x.cc, src/tl/remove_x.hh,
src/tl/simpfg.cc, src/tl/simpfg.hh, src/tl/simplify.cc,
src/tl/simplify.hh, src/tl/snf.cc, src/tl/snf.hh, src/tl/unabbrev.cc,
src/tl/unabbrev.hh, src/twa/bdddict.cc, src/twa/bdddict.hh,
src/twa/bddprint.cc, src/twa/formula2bdd.cc, src/twa/formula2bdd.hh,
src/twa/taatgba.cc, src/twa/taatgba.hh, src/twa/twa.hh,
src/twa/twagraph.cc, src/twa/twagraph.hh, src/twaalgos/compsusp.cc,
src/twaalgos/compsusp.hh, src/twaalgos/ltl2taa.cc,
src/twaalgos/ltl2taa.hh, src/twaalgos/ltl2tgba_fm.cc,
src/twaalgos/ltl2tgba_fm.hh, src/twaalgos/minimize.cc,
src/twaalgos/minimize.hh, src/twaalgos/neverclaim.cc,
src/twaalgos/postproc.cc, src/twaalgos/postproc.hh,
src/twaalgos/powerset.cc, src/twaalgos/powerset.hh,
src/twaalgos/randomgraph.cc, src/twaalgos/randomgraph.hh,
src/twaalgos/relabel.cc, src/twaalgos/relabel.hh,
src/twaalgos/remprop.cc, src/twaalgos/remprop.hh, src/twaalgos/stats.cc,
src/twaalgos/stats.hh, src/twaalgos/stutter.cc, src/twaalgos/stutter.hh,
src/twaalgos/translate.cc, src/twaalgos/translate.hh,
wrap/python/spot_impl.i: Remove the ltl namespace.

src/twa/bdddict.cc

@@ -95,7 +95,7 @@ namespace spot
   }
 
   int
-  bdd_dict::register_proposition(ltl::formula f, const void* for_me)
+  bdd_dict::register_proposition(formula f, const void* for_me)
   {
     int num;
     // Do not build a variable that already exists.
@@ -117,7 +117,7 @@ namespace spot
   }
 
   int
-  bdd_dict::has_registered_proposition(ltl::formula f,
+  bdd_dict::has_registered_proposition(formula f,
 				       const void* me)
   {
     auto ssi = var_map.find(f);
@@ -131,7 +131,7 @@ namespace spot
   }
 
   int
-  bdd_dict::register_acceptance_variable(ltl::formula f,
+  bdd_dict::register_acceptance_variable(formula f,
 					 const void* for_me)
   {
     int num;
@@ -171,7 +171,7 @@ namespace spot
     register_acceptance_variables(bdd_low(f), for_me);
   }
 
-  ltl::formula
+  formula
   bdd_dict::oneacc_to_formula(int var) const
   {
     assert(unsigned(var) < bdd_map.size());
@@ -180,7 +180,7 @@ namespace spot
     return i.f;
   }
 
-  ltl::formula
+  formula
   bdd_dict::oneacc_to_formula(bdd oneacc) const
   {
     assert(oneacc != bddfalse);
@@ -274,7 +274,7 @@ namespace spot
     // Let's free it.  First, we need to find
     // if this is a Var or an Acc variable.
     int n = 1;
-    ltl::formula f = nullptr;
+    formula f = nullptr;
     switch (bdd_map[v].type)
       {
       case var: