twa_graph: do not order BDDs by IDs in merge_edges()

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

@@ -76,6 +76,14 @@ New in spot 2.4.0.dev (not yet released)
     spot::scc_info::marks(), spot::scc_info::marks_of() and
     spot::scc_info::acc_sets_of() respectively.
 
+  Bugs fixed:
+
+  - The twa_graph::mege_edges() function relied on BDD IDs to sort
+    edges.  This in turn caused some algorithm (like the
+    degeneralization) to produce slighltly different outputs (but
+    still correct outputs) depending on the BDD operations performed
+    before.
+
 New in spot 2.4 (2017-09-06)
 
   Build: