scc_info: introduce edges_of() and inner_edges_of()

This is motivated by some upcoming patch by Heňo.

* spot/twaalgos/sccinfo.hh (edges_of, inner_edges_of): New methods.
* spot/twaalgos/sccinfo.cc, spot/twaalgos/strength.cc: Use them.
* spot/twa/twagraph.hh (edge_number): Add an overload.
* python/spot/impl.i: Bind the new methods.
* tests/python/sccinfo.py: Add tests.
* NEWS: Mention the changes.

spot/twa/twagraph.hh

@@ -349,6 +349,11 @@ namespace spot
       return i->pos();
     }
 
+    unsigned edge_number(const edge_storage_t& e) const
+    {
+      return g_.index_of_edge(e);
+    }
+
     twa_graph_edge_data& edge_data(const twa_succ_iterator* it)
     {
       return g_.edge_data(edge_number(it));