ocaml: phase 5.1 euler21_small.ml baseline (sum of amicable numbers <= 300 = 504)

div_sum computes proper divisor sum via trial division up to sqrt(n):

  let div_sum n =
    let s = ref 1 in
    let i = ref 2 in
    while !i * !i <= n do
      if n mod !i = 0 then begin
        s := !s + !i;
        let q = n / !i in
        if q <> !i then s := !s + q
      end;
      i := !i + 1
    done;
    if n = 1 then 0 else !s

Outer loop finds amicable pairs (a, b) with d(a) = b, d(b) = a,
a != b. Only pair under 300 is (220, 284); 220 + 284 = 504.

Real PE21 uses 10000 (answer 31626). 300 keeps the run under
budget while exercising the same divisor-sum trick.

119 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -30,6 +30,7 @@
   "euler10.ml": 1060,
   "euler14.ml": 97,
   "euler2.ml": 4613732,
+  "euler21_small.ml": 504,
   "euler25.ml": 55,
   "euler28.ml": 261,
   "euler30_cube.ml": 1301,