functionality of the old tgba_tba_proxy.
* src/tgba/tgbatba.cc (tgba_tba_proxy_succ_iterator,
tgba_tba_proxy): Rewrite to produce TBA with at most N copies of
each state, skipping the `bddtrue' stage now used only in
tgba_sba_proxy.  Doing so removes approximately 6% of states in
the degeneralized tests of spotlbtt.test.
(tgba_sba_proxy): Implement it.
* src/tgbaalgos/neverclaim.hh, src/tgbaalgos/neverclaim.cc: Adjust
to take a tgba_sba_proxy.
* src/tgbatest/ltl2tgba.cc: Add option -DS and adjust call to
never_claim_reachable().