ocaml: phase 5.1 frequency.ml baseline + Format module alias (+2 tests, 498 total)

frequency.ml exercises the recently-added Hashtbl.iter / fold +
Hashtbl.find_opt + s.[i] indexing + for-loop together: build a
char-count table for 'abracadabra' then take the max via
Hashtbl.fold. Expected = 5 (a x 5). Total 25 baseline programs.

Format module added as a thin alias of Printf — sprintf, printf, and
asprintf all delegate to Printf.sprintf. The dynamic runtime doesn't
distinguish boxes/breaks, so format strings work the same as in
Printf and most Format-using OCaml programs now compile.

lib/ocaml/baseline/frequency.ml

@@ -0,0 +1,18 @@
+let count_chars s =
+  let t = Hashtbl.create 8 in
+  for i = 0 to String.length s - 1 do
+    let c = s.[i] in
+    let n = match Hashtbl.find_opt t c with
+            | Some v -> v + 1
+            | None -> 1
+    in
+    Hashtbl.replace t c n
+  done;
+  t
+
+let max_count t =
+  Hashtbl.fold (fun _ v acc -> if v > acc then v else acc) t 0
+
+;;
+
+max_count (count_chars "abracadabra")