sort: improve average quicksort performance · golang/go@6f6b2f0

- change way of protection from O(N^2) on duplicate values. Previous algorithm does additional comparisons and swaps on every split pass. Changed algorithm does one ordinal quicksort split pass, and if distribution is skewed, then additional pass to separate pivot's duplicates. Changed algorithm could be slower on very ununique slice, but it is still protected from O(N^2). - increase small slice s