ocaml: phase 5.1 monotonic.ml baseline (monotonicity check, 4/5 inputs monotonic)

Tracks two bool refs (inc, dec). For each pair of consecutive
elements: if h < prev clear inc, if h > prev clear dec. Returns
inc OR dec at the end:

  let is_monotonic xs =
    match xs with
    | [] -> true
    | [_] -> true
    | _ ->
      let inc = ref true in
      let dec = ref true in
      let rec walk prev rest = ... in
      (match xs with h :: t -> walk h t | [] -> ());
      !inc || !dec

Five test cases:
  [1;2;3;4]    inc only        true
  [4;3;2;1]    dec only        true
  [1;2;1]      neither          false
  [5;5;5]      both (constant)  true
  []            empty            true (vacuous)
  sum = 4

98 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -59,6 +59,7 @@
   "merge_sort.ml": 44,
   "merge_two.ml": 441,
   "module_use.ml": 3,
+  "monotonic.ml": 4,
   "newton_sqrt.ml": 1414,
   "mutable_record.ml": 10,
   "option_match.ml": 5,