Let's use Arturs's quicksort as the fallback sort
Fixed the bounds a little

I doubt we'll be sorting anything besides US-ASCII, and the theoretically
optimal bound for falling back to a comparison sort is the square root of
the size of the alphabet; this should probably be verified in practice,
though.
Removed useless diagnostic function
Commented these out since they were messing with my test output

You really shouldn't put testing code in the main block, Arturs
Implemented LSD radix sort

But it's slow as all hell: just sorting `dna.200MB` took two and a half
minutes.  Perhaps it ought to be dropped.