python: improve kripke_graph bindings

Related to issue #376.

* spot/kripke/kripkegraph.hh: Avoid indirect type definitions for the
benefit of Swig.
* python/spot/impl.i: Add bindings for iterators over kripke_graph
states and edges.
* tests/python/kripke.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Update.

tests/Makefile.am

@@ -372,7 +372,6 @@ TESTS_python = \
   python/bugdet.py \
   python/complement_semidet.py \
   python/declenv.py \
-  python/_word.ipynb \
   python/decompose_scc.py \
   python/dualize.py \
   python/except.py \
@@ -380,6 +379,7 @@ TESTS_python = \
   python/genem.py \
   python/implies.py \
   python/interdep.py \
+  python/kripke.py \
   python/ltl2tgba.test \
   python/ltlf.py \
   python/ltlparse.py \
@@ -417,6 +417,7 @@ TESTS_python = \
   python/tra2tba.py \
   python/twagraph.py \
   python/toweak.py \
+  python/_word.ipynb \
   $(TESTS_ipython)
 endif