tgba_succ_iterator) with some string.  This comes handy to
associate that transition to its high-level name.
* src/tgba/tgba.hh, src/tgba/tgba.cc (tgba::transition_annotation):
New method.
* src/tgba/tgbaproduct.hh, src/tgba/tgbaproduct.cc
(tgba_product::transition_annotation): Implement it.
* src/tgba/tgbatba.hh, src/tgba/tgbatba.cc
(tgba_tba_proxy::transition_annotation): Likewise.
* src/tgbaalgos/replayrun.cc (print_annotation): New function.
(replay_tgba_run): Use it.