ocaml: phase 5.1 euler25.ml baseline (first 12-digit Fibonacci index = 55)

Iteratively grows two refs while the larger is below 10^(n-1),
counting iterations:

  let euler25 n =
    let a = ref 1 in
    let b = ref 1 in
    let i = ref 2 in
    let target = ref 1 in
    for _ = 1 to n - 1 do target := !target * 10 done;
    while !b < !target do
      let c = !a + !b in
      a := !b;
      b := c;
      i := !i + 1
    done;
    !i

  euler25 12 = 55   (F(55) = 139_583_862_445, 12 digits)

Real PE25 asks for 1000 digits (answer 4782); 12 keeps within
safe-int while exercising the identical algorithm.

114 baseline programs total — 200 iterations landed.

lib/ocaml/baseline/euler25.ml

@@ -0,0 +1,17 @@
+let euler25 n =
+  let a = ref 1 in
+  let b = ref 1 in
+  let i = ref 2 in
+  let target = ref 1 in
+  for _ = 1 to n - 1 do target := !target * 10 done;
+  while !b < !target do
+    let c = !a + !b in
+    a := !b;
+    b := c;
+    i := !i + 1
+  done;
+  !i
+
+;;
+
+euler25 12

lib/ocaml/baseline/expected.json

@@ -28,6 +28,7 @@
   "euler1.ml": 233168,
   "euler10.ml": 1060,
   "euler2.ml": 4613732,
+  "euler25.ml": 55,
   "euler3.ml": 29,
   "euler4_small.ml": 9009,
   "euler5.ml": 232792560,