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 implementation
* python/spot/impl.i: Add dualize() to python interface.
* spot/twaalgos/Makefile.am: Add dualize.cc,hh to the build
* spot/twaalgos/dualize.cc: Implement dualize() that takes an automaton
and returns its dual
* spot/twaalgos/dualize.hh: Implement dualize()
* tests/Makefile.am: Add dualize tests to the test suite
* tests/python/dualize.py: Test cases for dualize
