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/runtime.sx

@@ -447,6 +447,11 @@
          | h :: t -> aux t (acc ^ h)
        in
        aux xs \"\"
+     let equal a b = a = b
+     let compare a b =
+       if a < b then -1 else if a > b then 1 else 0
+     let cat a b = a ^ b
+     let empty = \"\"
    end ;;
 
    module Bytes = struct