stats: fix slow %s and inappropriate %S output

Fixes #269.

* spot/twaalgos/stats.cc: Use twa_statistics instead of
twa_sub_statistics when %t is not used.
* bin/common_aoutput.cc: Likewise, also fix %S to use twa_statistics
instead of num_states(), and document that %s,%t,%e all return
statistics about the reachable part of the automaton.
* tests/core/format.test: Add more tests.
* NEWS: Document the issue.

spot/twaalgos/stats.cc

@@ -346,7 +346,7 @@ namespace spot
       }
     else if (has('s') || has('e'))
       {
-        twa_sub_statistics s = sub_stats_reachable(aut);
+        twa_statistics s = stats_reachable(aut);
         states_ = s.states;
         edges_ = s.edges;
       }