i_viterbi(): iterative Viterbi (pruning not implemented yet). Also,
--excludeFirstOrder option.
quash EVIL BUG
pruning for iterative Viterbi!
iterative Viterbi: actually enable pruning and apply some other
optimizations. but much slower than conventional Viterbi when
considering label bigrams (with --bio or without --excludeFirstOrder);
perhaps first-order scores for degenerate label pairs need to be cached
as well
i_viterbi(): properly cache bigram scores when there are degenerate
labels