ocaml: phase 5.1 palindrome.ml baseline (two-pointer check, 4/6 inputs match)

Two-pointer palindrome check:

  let is_palindrome s =
    let n = String.length s in
    let rec check i j =
      if i >= j then true
      else if s.[i] <> s.[j] then false
      else check (i + 1) (j - 1)
    in
    check 0 (n - 1)

Tests on six strings:
  racecar = true
  hello   = false
  abba    = true
  ''      = true (vacuously, i >= j on entry)
  'a'     = true
  'ab'    = false

Sum = 4.

Uses s.[i] <> s.[j] (string-get + structural inequality), recursive
2-arg pointer advancement, and a multi-clause if/else if/else for
the three cases.

48 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -31,6 +31,7 @@
   "newton_sqrt.ml": 1414,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
+  "palindrome.ml": 4,
   "pascal.ml": 252,
   "pi_leibniz.ml": 314,
   "pretty_table.ml": 64,