Investigate further optimizations of the algorithm

[hiddentao]

We don't need to store the current row, just the last value calculated in the current row. Implement this optimization.

[wooorm]

Just wanted you to know that I've implemented this enhancement in my levenshtein module – It was a bit hard to bend by brain around, but in the end it worked out great. You should implement it too!

[hiddentao]

@wooorm Thanks. I've added in this optimization and imported your benchmark tests (please see the README.md for results).

[wooorm]

Nice job, although bittersweet: now I have to make my module faster! 👍