Optimize levenshtein a bit for memory usage

[nielsdos]

When all costs are equal, levenshtein fulfills the requirements of being a metric. A metric is symmetric, so we can swap the strings in that case. Since we use rows of a partial matrix of length |string2| we can make the choice of using string1 instead if |string1| < |string2|, which will optimize memory usage and CPU time.

Sample bench script:

<?php
for ($i = 0;$i<2000;$i++)
    levenshtein(str_repeat('a', 100), str_repeat('b', 1000), 1, 1, 1);

results:

Benchmark 1: ./sapi/cli/php x.php 
  Time (mean ± σ):     254.5 ms ±   1.7 ms    [User: 251.5 ms, System: 2.2 ms]
  Range (min … max):   252.5 ms … 257.4 ms    11 runs
 
Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     292.0 ms ±   2.7 ms    [User: 289.8 ms, System: 1.8 ms]
  Range (min … max):   287.4 ms … 295.6 ms    10 runs
 
Summary
  ./sapi/cli/php x.php  ran
    1.15 ± 0.01 times faster than ./sapi/cli/php_old x.php

[bukka]

I haven't thought too much about this but it makes sense to me.