fix typo + faster is_sorted

[Wireless4024]

• Fix typo : InmutableLessSwap -> ImmutableLessSwap

• IndexSort::is_sorted2()
  This is an improved version of IndexSort::is_sorted.
  I didn't replace internal function with is_sorted2 because I'm not sure about its behavior,
  If you want to replace original function with this new implementation, do it manually (or tell me to do so).

  worst case scenario: O(N/2)

  This implementation compare element at head and tail in same loop (visualize below)

  len = 13
  iter 1: less(1, 0), less(12, 11)
  iter 2: less(2, 1), less(11, 10)
  iter 3: less(3, 2), less(10, 9)
  iter 4: less(4, 3), less(9, 8)
  iter 5: less(5, 4), less(8, 7)
  iter 6: less(6, 5), less(7, 6)
  

  if len is even it will compare middle element twice (less(7, 6) will run twice if len=14)

  result from cargo bench

   is_sorted(10k)          time:   [8.9054 µs 8.9057 µs 8.9059 µs]
   1 (1.00%) low severe
   2 (2.00%) high mild
   2 (2.00%) high severe
  
   is_sorted2(10k)         time:   [6.1242 µs 6.1258 µs 6.1283 µs]
   1 (1.00%) low severe
   2 (2.00%) low mild
   1 (1.00%) high mild
   8 (8.00%) high severe
  
   is_sorted(100m)         time:   [115.44 ms 117.43 ms 119.60 ms]
   1 (1.00%) high mild
  
   is_sorted2(100m)        time:   [84.528 ms 84.769 ms 84.978 ms]
   5 (5.00%) low severe
   3 (3.00%) low mild
   1 (1.00%) high mild