The ltl prefix does not make a lot of sens anymore (since we
support psl as well).  ltlast/ and ltlenv/ were almost empty.
And ltlvisit/ did not contain any visitor anymore.

* src/ltlvisit/, src/ltlast/, src/ltlenv/: Merge into...
* src/tl/: ...this.
* NEWS: Mention the change.
* README, bench/stutter/stutter_invariance_formulas.cc,
bench/stutter/stutter_invariance_randomgraph.cc, configure.ac,
doc/org/tut01.org, doc/org/tut02.org, doc/org/tut10.org, doc/tl/tl.tex,
iface/ltsmin/ltsmin.hh, iface/ltsmin/modelcheck.cc, src/Makefile.am,
src/bin/autfilt.cc, src/bin/common_output.cc, src/bin/common_output.hh,
src/bin/common_r.hh, src/bin/common_trans.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/randltl.cc, src/kripke/kripkeexplicit.hh,
src/kripkeparse/public.hh, src/parseaut/public.hh, src/priv/accmap.hh,
src/ta/taexplicit.hh, src/ta/tgtaexplicit.hh, src/tests/equalsf.cc,
src/tests/ikwiad.cc, src/tests/length.cc, src/tests/ltlrel.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/twagraph.cc, src/twa/acc.hh, src/twa/bdddict.cc,
src/twa/bdddict.hh, src/twa/bddprint.cc, src/twa/taatgba.cc,
src/twa/taatgba.hh, src/twa/twa.hh, src/twa/twagraph.cc,
src/twa/twagraph.hh, src/twa/twasafracomplement.cc,
src/twaalgos/compsusp.cc, src/twaalgos/compsusp.hh,
src/twaalgos/dtgbasat.cc, src/twaalgos/hoa.cc,
src/twaalgos/isweakscc.cc, src/twaalgos/lbtt.cc,
src/twaalgos/ltl2taa.cc, src/twaalgos/ltl2taa.hh,
src/twaalgos/ltl2tgba_fm.cc, src/twaalgos/ltl2tgba_fm.hh,
src/twaalgos/minimize.hh, src/twaalgos/neverclaim.cc,
src/twaalgos/randomgraph.hh, src/twaalgos/relabel.hh,
src/twaalgos/remprop.hh, src/twaalgos/stats.cc, src/twaalgos/stutter.cc,
src/twaalgos/translate.hh, wrap/python/spot_impl.i,
src/ltlparse/ltlparse.yy, src/ltlparse/public.hh: Adjust.