sccinfo: improve detection of rejecting 1-self-loop SCCs
As observed in #188, the smaller.hoa automaton is made only of 1-state/1-self-loop SCCs, for which calling remove_fin is a complete waste of time. This patch alone (i.e., without the other changes suggested by #188) improves the run time of % autofilt -q --is-unambiguous smaller.hoa from 38s to 0.05s. * spot/twaalgos/sccinfo.cc: If a single-state SCC has undeterminate SCC and only one self-loop, then it is necessarily rejecting. * NEWS: Mention the change.