ocaml: phase 5.1 expr_eval.ml baseline (9/9 pass)

A tiny arithmetic-expression evaluator using:
  type expr = Lit of int | Add of expr*expr | Mul of expr*expr | Neg of expr
  let rec eval e = match e with | Lit n -> n | Add (a,b) -> ...

Exercises type-decl + multi-arg ctor + recursive match end-to-end.
Per-program timeout in run.sh bumped to 120s.

lib/ocaml/baseline/expected.json

@@ -1,6 +1,7 @@
 {
   "closures.ml": 315,
   "exception_handle.ml": 4,
+  "expr_eval.ml": 16,
   "factorial.ml": 3628800,
   "list_ops.ml": 30,
   "module_use.ml": 3,