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.

NEWS

@@ -28,9 +28,9 @@ New in spot 1.99.3a (not yet released)
   * The class hierarchy for temporal formulas has been entirely
     rewritten.  This change is actually quite massive (~13200 lines
     removed, ~8200 lines added), and brings some nice benefits:
-    - LTL/PSL formulas are now represented by lightweight ltl::formula
-      objects (instead of ltl::formula* pointers) that perform
-      reference counting automatically.
+    - LTL/PSL formulas are now represented by lightweight formula
+      objects (instead of pointers to children of an abstract formula
+      class) that perform reference counting automatically.
     - There is no hierachy anymore: all operators are represented by a
       single type of node in the syntax tree, and an enumerator is
       used to distinguish between operators.
@@ -41,10 +41,13 @@ New in spot 1.99.3a (not yet released)
       more friendly, and several algorithms that spanned a few pages
       have been reduced to a few lines.
 
-  * The source directories ltlast/, ltlenv/, and ltlvisit/, have been
-    merged into a single tl/ directory (for temporal logic).  This is
-    motivated by the fact that these formulas are not restricted to
-    LTL, and by the fact that we no longuer use the "visitor" pattern.
+  * Directories ltlast/, ltlenv/, and ltlvisit/, have been merged into
+    a single tl/ directory (for temporal logic).  This is motivated by
+    the fact that these formulas are not restricted to LTL, and by the
+    fact that we no longuer use the "visitor" pattern.
+
+  * For similar reasons, the spot::ltl namespace has been merged
+    with the spot namespace.
 
 New in spot 1.99.3 (2015-08-26)