ocaml: phase 5.1 luhn.ml baseline (Luhn check-digit, 2/4 inputs valid)

Walks digits right-to-left, doubles every other starting from the
second-from-right; if a doubled value > 9, subtract 9. Sum must be
divisible by 10:

  let luhn s =
    let n = String.length s in
    let total = ref 0 in
    for i = 0 to n - 1 do
      let d = Char.code s.[n - 1 - i] - Char.code '0' in
      let v = if i mod 2 = 1 then
        let dd = d * 2 in
        if dd > 9 then dd - 9 else dd
      else d
      in
      total := !total + v
    done;
    !total mod 10 = 0

Test cases:
  '79927398713'        valid
  '79927398710'        invalid
  '4532015112830366'   valid (real Visa test)
  '1234567890123456'   invalid
  sum = 2

Tests right-to-left index walk via 'n - 1 - i', Char.code '0'
arithmetic for digit conversion, and nested if-then-else.

75 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -35,6 +35,7 @@
   "fizzbuzz.ml": 57,
   "flatten_tree.ml": 28,
   "list_ops.ml": 30,
+  "luhn.ml": 2,
   "mat_mul.ml": 621,
   "max_path_tree.ml": 11,
   "mod_inverse.ml": 27,