Previously, it ignored 'start', sorting from the array's
beginning. This caused unintended changes and slower
performance. Fix ensures sorting stays within 'start'
and 'end' indices only.
Implement insertion sort in AK. The cutoff value 7 is a magic number
here, values [5, 15] should work well. Main idea of the cutoff is to
reduce recursion performed by quicksort to speed up sorting
of small partitions.