NewRadixSelector Sole constructor.

HISTOGRAM_SIZE size of histograms: 256 + 1 to indicate that the string is finished.

LENGTH_THRESHOLD buckets below this size will be sorted with introselect.

LEVEL_THRESHOLD after that many levels of recursion we fall back to introselect anyway this is used as a protection against the fact that radix sort performs worse when there are long common prefixes (probably because of cache locality).