ocaml: phase 5.1 merge_sort.ml baseline (user mergesort, sum=44)

User-implemented mergesort that exercises features added across the
last few iterations:

  let rec split lst = match lst with
    | x :: y :: rest ->
      let (a, b) = split rest in       (* iter 98 let-tuple destruct *)
      (x :: a, y :: b)
    | ...

  let rec merge xs ys = match xs with
    | x :: xs' ->
      match ys with                     (* nested match-in-match *)
      | y :: ys' -> ...

  ...

  List.fold_left (+) 0 (sort [...])     (* iter 89 (op) section *)

Sum of [3;1;4;1;5;9;2;6;5;3;5] = 44 regardless of order, so the
result is also a smoke test of the implementation correctness — if
merge_sort drops or duplicates an element the sum diverges. 26
baseline programs total.

lib/ocaml/baseline/expected.json

@@ -16,6 +16,7 @@
   "lambda_calc.ml": 7,
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
+  "merge_sort.ml": 44,
   "module_use.ml": 3,
   "mutable_record.ml": 10,
   "option_match.ml": 5,