ocaml: phase 5.1 prefix_sum.ml baseline (precomputed sums + range queries, 14+25+27 = 66)

Two utility functions:

  prefix_sums xs   builds Array of len n+1 such that
                   arr.(i) = sum of xs[0..i-1]
  range_sum p lo hi = p.(hi+1) - p.(lo)

For [3;1;4;1;5;9;2;6;5;3]:
  range_sum 0 4   = 14   (3+1+4+1+5)
  range_sum 5 9   = 25   (9+2+6+5+3)
  range_sum 2 7   = 27   (4+1+5+9+2+6)
  sum             = 66

Tests List.iter mutating Array indexed by a ref counter, plus the
classic prefix-sum technique for O(1) range queries.

67 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -47,6 +47,7 @@
   "palindrome.ml": 4,
   "pascal.ml": 252,
   "pi_leibniz.ml": 314,
+  "prefix_sum.ml": 66,
   "pretty_table.ml": 64,
   "poly_stack.ml": 5,
   "prime_factors.ml": 17,