ocaml: phase 5.1 expr_simp.ml baseline (symbolic simplifier, eval(simp e) = 22)

Recursive ADT with three constructors (Num/Add/Mul). simp does
bottom-up rewrite using algebraic identities:

  x + 0  -> x
  0 + x  -> x
  x * 0  -> 0
  0 * x  -> 0
  x * 1  -> x
  1 * x  -> x
  constant folding for Num + Num and Num * Num

Uses tuple pattern in nested match: 'match (simp a, simp b) with'.

  Add (Mul (Num 3, Num 5), Add (Num 0, Mul (Num 1, Num 7)))
   -> simp ->  Add (Num 15, Num 7)
   -> eval ->  22

51 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -15,6 +15,7 @@
   "csv.ml": 10,
   "exception_handle.ml": 4,
   "expr_eval.ml": 16,
+  "expr_simp.ml": 22,
   "factorial.ml": 3628800,
   "fraction.ml": 7,
   "frequency.ml": 5,