ocaml: phase 5.1 fraction.ml baseline (rational arithmetic, 4/3 -> num+den=7)

Defines:

  type frac = { num : int; den : int }
  let rec gcd a b = if b = 0 then a else gcd b (a mod b)
  let make n d =                        (* canonicalise: gcd-reduce and
                                           force den > 0 *)
  let add x y = make (x.num * y.den + y.num * x.den) (x.den * y.den)
  let mul x y = make (x.num * y.num) (x.den * y.den)

Test:
  let r = add (make 1 2) (make 1 3) in     (* 5/6 *)
  let s = mul (make 2 3) (make 3 4) in     (* 1/2 *)
  let t = add r s in                       (* 5/6 + 1/2 = 4/3 *)
  t.num + t.den                            (* = 7 *)

Exercises records, recursive gcd, mod, abs, integer division (the
truncate-toward-zero semantics from iter 94 are essential here —
make would diverge from real OCaml's behaviour with float division).
28 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -10,6 +10,7 @@
   "exception_handle.ml": 4,
   "expr_eval.ml": 16,
   "factorial.ml": 3628800,
+  "fraction.ml": 7,
   "frequency.ml": 5,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,