ocaml: phase 5.1 hist.ml baseline (Hashtbl int histogram, total * max = 75)

Three small functions:

  hist xs       build a Hashtbl of count-by-value
  max_value h   Hashtbl.fold to find the max bin
  total h       Hashtbl.fold to sum all bins

For the 15-element list [1;2;3;1;4;5;1;2;6;7;1;8;9;1;0]:
  total      = 15
  max_value  =  5 (the number 1 appears 5 times)
  product    = 75

Companion to bag.ml (string keys) and frequency.ml (char keys) —
same Hashtbl.fold + Hashtbl.find_opt pattern, exercised on int
keys this time.

71 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -31,6 +31,7 @@
   "group_consec.ml": 53,
   "hailstone.ml": 111,
   "hanoi.ml": 1023,
+  "hist.ml": 75,
   "fizzbuzz.ml": 57,
   "flatten_tree.ml": 28,
   "list_ops.ml": 30,