ocaml: phase 5.1 bag.ml baseline + String.equal/compare/cat/empty (+3 tests, 579 total)

bag.ml: split a sentence on spaces, count each word in a Hashtbl,
return the maximum count via Hashtbl.fold.

  count_words 'the quick brown fox jumps over the lazy dog the fox'
  -> Hashtbl with 'the' = 3 as the max
  -> 3

Exercises String.split_on_char + Hashtbl.find_opt/replace +
Hashtbl.fold (k v acc -> ...). Together with frequency.ml from
iter 84 we now have two Hashtbl-counting baselines exercising
slightly different idioms. 29 baseline programs total.

String additions:
  equal a b      = a = b
  compare a b    = -1 / 0 / 1 via host < / >
  cat a b        = a ^ b
  empty          = '' (constant)

lib/ocaml/baseline/bag.ml

@@ -0,0 +1,18 @@
+let count_words text =
+  let words = String.split_on_char ' ' text in
+  let counts = Hashtbl.create 8 in
+  List.iter (fun w ->
+    let n = match Hashtbl.find_opt counts w with
+            | Some n -> n + 1
+            | None -> 1
+    in
+    Hashtbl.replace counts w n
+  ) words;
+  counts
+
+let max_count counts =
+  Hashtbl.fold (fun _ v acc -> if v > acc then v else acc) counts 0
+
+;;
+
+max_count (count_words "the quick brown fox jumps over the lazy dog the fox")

lib/ocaml/baseline/expected.json

@@ -1,5 +1,6 @@
 {
   "anagrams.ml": 3,
+  "bag.ml": 3,
   "bfs.ml": 6,
   "btree.ml": 39,
   "brainfuck.ml": 75,