ocaml: phase 5.1 atm.ml baseline (mutable record + exception + try/with, balance = 120)

Models a bank account using a mutable record + a user exception:

  type account = { mutable balance : int }
  exception Insufficient

  let withdraw acct amt =
    if amt > acct.balance then raise Insufficient
    else acct.balance <- acct.balance - amt

Sequence:
  start             100
  deposit 50        150
  withdraw 30       120
  withdraw 200      raises Insufficient
  handler returns   acct.balance        (= 120, transaction rolled back)

Combines mutable record fields, user exception declaration,
try-with-bare-pattern, and verifies that a raise in the middle of a
sequence doesn't leave a partial mutation.

59 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -2,6 +2,7 @@
   "ackermann.ml": 125,
   "anagram_check.ml": 2,
   "anagrams.ml": 3,
+  "atm.ml": 120,
   "bag.ml": 3,
   "bf_full.ml": 6,
   "bigint_add.ml": 28,