ocaml: phase 5.1 word_freq.ml baseline (Map.Make on String, distinct = 8)

First baseline using Map.Make on a string-keyed map:

  module StringOrd = struct
    type t = string
    let compare = String.compare
  end
  module SMap = Map.Make (StringOrd)

  let count_words text =
    let words = String.split_on_char ' ' text in
    List.fold_left (fun m w ->
      let n = match SMap.find_opt w m with
              | Some n -> n
              | None -> 0
      in
      SMap.add w (n + 1) m
    ) SMap.empty words

For 'the quick brown fox jumps over the lazy dog' ('the' appears
twice), SMap.cardinal -> 8.

Complements bag.ml (Hashtbl-based) and unique_set.ml (Set.Make)
with a sorted Map view of the same kind of counting problem. 35
baseline programs total.

lib/ocaml/baseline/expected.json

@@ -31,6 +31,7 @@
   "roman.ml": 44,
   "safe_div.ml": 20,
   "shuffle.ml": 55,
+  "word_freq.ml": 8,
   "sieve.ml": 15,
   "sum_squares.ml": 385,
   "unique_set.ml": 9,