Fixes#282.
* spot/misc/bddlt.hh (bdd_less_than_stable): New function.
* spot/twa/twagraph.cc (merge_edges): Use it.
* tests/core/genltl.test: Adjust, and add an extra test
for the behavior of #282.
* tests/core/complement.test, tests/core/degenid.test,
tests/core/ltldo.test, tests/core/prodor.test,
tests/core/readsave.test, tests/core/sbacc.test,
tests/python/atva16-fig2a.ipynb, tests/python/automata.ipynb,
tests/python/decompose.ipynb, tests/python/dualize.py,
tests/python/highlighting.ipynb, tests/python/piperead.ipynb,
tests/python/product.ipynb, tests/python/simstate.py,
tests/python/tra2tba.py: Adjust all expected outputs.
* NEWS: Mention the bug.
* NEWS: Mention the change.
* src/: Rename as ...
* spot/: ... this, adjust all headers to include <spot/...> instead of
"...", and adjust all Makefile.am to search headers from the top-level
directory.
* HACKING: Add conventions about #include.
* spot/sanity/style.test: Add a few more grep to catch cases
that do not follow these conventions.
* .gitignore, Makefile.am, README, bench/stutter/Makefile.am,
bench/stutter/stutter_invariance_formulas.cc,
bench/stutter/stutter_invariance_randomgraph.cc, configure.ac,
debian/rules, doc/Doxyfile.in, doc/Makefile.am,
doc/org/.dir-locals.el.in, doc/org/g++wrap.in, doc/org/init.el.in,
doc/org/tut01.org, doc/org/tut02.org, doc/org/tut03.org,
doc/org/tut10.org, doc/org/tut20.org, doc/org/tut21.org,
doc/org/tut22.org, doc/org/tut30.org, iface/ltsmin/Makefile.am,
iface/ltsmin/kripke.test, iface/ltsmin/ltsmin.cc,
iface/ltsmin/ltsmin.hh, iface/ltsmin/modelcheck.cc,
wrap/python/Makefile.am, wrap/python/ajax/spotcgi.in,
wrap/python/spot_impl.i, wrap/python/tests/ltl2tgba.py,
wrap/python/tests/randgen.py, wrap/python/tests/run.in: Adjust.
