Minor optimizations to classical_lcs().
Minor optimizations to classical_lcs().

Use a while loop to cut out some deep recursion.

When constructing the vector "ta", omit any lines that are
already known not to appear in "b"; these lines cannot appear
in LCS(a,b). We could even more productively prune lines from
"b" that don't appear in "ta"; but right now we're getting away
with not making a copy of "b" at all, and constructing a pruned
copy might hurt more than skipping the pruning altogether.

Quuxplusone/difdef,Quuxplusone/difdef,Quuxplusone/difdef