ocaml: phase 5.1 count_inversions.ml baseline (12 inversions via merge sort)

Modified merge sort that counts inversions during the merge step:
when an element from the right half is selected, the remaining
elements of the left half (mid - i + 1) all form inversions with
that right element.

  count_inv [|8; 4; 2; 1; 3; 5; 7; 6|] = 12

Inversions of [8;4;2;1;3;5;7;6]:
  with 8: (8,4)(8,2)(8,1)(8,3)(8,5)(8,7)(8,6) = 7
  with 4: (4,2)(4,1)(4,3) = 3
  with 2: (2,1) = 1
  with 7: (7,6) = 1
                                        total = 12

Tests: let rec ... and ... mutual recursion, while + ref + array
mutation, in-place sort with auxiliary scratch array.

145 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -24,6 +24,7 @@
   "closures.ml": 315,
   "coin_change.ml": 6,
   "count_change.ml": 406,
+  "count_inversions.ml": 12,
   "csv.ml": 10,
   "dijkstra.ml": 7,
   "exception_handle.ml": 4,