ocaml: phase 5.1 rpn.ml baseline (Reverse Polish Notation evaluator, [3 4 + 2 * 5 -] = 9)

Stack-based RPN evaluator:

  let eval_rpn tokens =
    let stack = Stack.create () in
    List.iter (fun tok ->
      if tok is operator then
        let b = Stack.pop stack in
        let a = Stack.pop stack in
        Stack.push (apply tok a b) stack
      else
        Stack.push (int_of_string tok) stack
    ) tokens;
    Stack.pop stack

For tokens [3 4 + 2 * 5 -]:
  3 4 +  ->  7
  7 2 *  ->  14
  14 5 - ->  9

Exercises Stack.create / push / pop, mixed branch on string
equality, multi-arm if/else if for operator dispatch, int_of_string
for token parsing.

39 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -31,6 +31,7 @@
   "queens.ml": 2,
   "quicksort.ml": 44,
   "roman.ml": 44,
+  "rpn.ml": 9,
   "safe_div.ml": 20,
   "shuffle.ml": 55,
   "word_freq.ml": 8,