With this change the output of
ltl2tgba -f -x -k -DS "GF(p_1) & ... & GF(p_n)
uses less than (n+1)^2 transitions when it used
exactly (n+1)*(2^n) transitions before.

* src/tgba/tgbatba.cc (tgba_tba_proxy_succ_iterator): Merge
transitions going to the same states if they are both accepting or
if neither are.
(state_ptr_bool_t, state_ptr_bool_less_than): Helper type to
store a transition in tgba_tba_proxy_succ_iterator.
* src/tgba/tgbatba.cc, src/tgba/tgbatba.hh
(tgba_tba_proxy::transition_annotation): Remove.  We cannot
implement this method if transitions are merged.