Shell sort on [22 10 15 3 8 2] with gaps 3, 1
  after gap 3: [3 8 2 22 10 15]
  after gap 1: [2 3 8 10 15 22]
  comparisons = 11
Comparisons on 100000 random numbers:
  Shell n/2: 16 gaps, 4488726 comparisons
  Knuth 3h+1: 11 gaps, 3842607 comparisons
  Ciura: 13 gaps, 2554426 comparisons
