safety fix

--HG--
extra : amend_source : 280d9948d6fd20f50c54d7fe35c72f78012e8e46
really only read from C file when seek position is 0
minor cythonisations
speed up group_transitions_by_state()
speed up nfa2dfa() computation and prune unreachable states
changelog
minor tweak to reduce code overhead
fix debug code in Py3
remove some more overhead from nfa2dfa()