never iterate on the edge_vector()
This fixes #552 (closed), reported by Rüdiger and Ayrat.
- tests/sanity/style.test: Warn aginst iterations on edge_vector.
- spot/parseaut/parseaut.yy, spot/twaalgos/complete.cc, spot/twaalgos/parity.cc: Iterate over edges(), not edge_vector().
- tests/core/ltlcross.test: Add a test case for #552 (closed).
- NEWS: Mention the bug.