This involves reimplementing some algorithms using tgba_digraph, and
implementing an explicit product that takes two tgba_digraphs and
produces a tgba_digraph.

* src/tgbaalgos/product.cc, src/tgbaalgos/product.hh: New files.
* src/tgbaalgos/Makefile.am: Adjust.
* src/bin/ltlcross.cc, src/tgba/tgba.cc, src/tgba/tgba.hh,
src/tgba/tgbasafracomplement.cc, src/tgba/tgbasafracomplement.hh,
src/tgbaalgos/cycles.cc, src/tgbaalgos/cycles.hh,
src/tgbaalgos/degen.cc, src/tgbaalgos/degen.hh,
src/tgbaalgos/isweakscc.cc, src/tgbaalgos/isweakscc.hh,
src/tgbaalgos/minimize.cc, src/tgbaalgos/minimize.hh,
src/tgbaalgos/powerset.cc, src/tgbaalgos/powerset.hh,
src/tgbaalgos/safety.cc, src/tgbaalgos/safety.hh,
src/tgbaalgos/sccinfo.cc, src/tgbaalgos/sccinfo.hh,
src/tgbatest/complementation.cc, src/tgbatest/emptchk.cc,
src/tgbatest/ltl2ta.test, src/tgbatest/ltl2tgba.cc,
src/tgbatest/randtgba.cc: Update to use scc_info and/or tgba_digraph.