ocaml: phase 5.1 palindrome_sum.ml baseline (sum of 3-digit palindromes = 49500)

let palindrome_sum lo hi =
    let total = ref 0 in
    for n = lo to hi do
      if is_pal n then total := !total + n
    done;
    !total

  palindrome_sum 100 999 = 49500

There are 90 three-digit palindromes (form aba; 9 choices for a, 10
for b). Average value 550, sum 49500.

Companion to palindrome.ml (predicate-only) and paren_depth.ml.

127 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -89,6 +89,7 @@
   "mutable_record.ml": 10,
   "option_match.ml": 5,
   "palindrome.ml": 4,
+  "palindrome_sum.ml": 49500,
   "paren_depth.ml": 7,
   "partition.ml": 3025,
   "pancake_sort.ml": 910,