* src/tgbaalgos/sccinfo.cc, src/tgbaalgos/sccinfo.hh: add
scc_info::begin() and scc_info::end() methods to iterate over the
node_ vector.   Tidy the scc_node structure that that its member
are accessed via methods.
* src/tgbaalgos/safety.cc, src/bin/ltlcross.cc: Simplify using
this interface.