stats: speed up the computation of transitions
Juraj Major reported a case with 32 APs where ltlcross would take forever to gather statistics. It turns out that for each edge, twa_sub_statistics was enumerating all compatible assignments of 32 APs. This uses bdd_satcountset() instead, and also store the result in a long long to avoid overflows. * spot/twaalgos/stats.cc (twa_sub_statistics): Improve the code for counting transitions. * bin/common_aoutput.hh, bin/ltlcross.cc, spot/twaalgos/stats.hh: Store transition counts are long long. * tests/core/readsave.test: Add test case. * NEWS: Mention the bug.