ocaml: phase 5.1 json_pretty.ml baseline (recursive ADT to string, len = 24)

Defines a JSON-like algebraic data type:

  type json =
    | JNull
    | JBool of bool
    | JInt of int
    | JStr of string
    | JList of json list

Recursively serialises to a string via match-on-constructor, then
measures the length:

  JList [JInt 1; JBool true; JNull; JStr 'hi'; JList [JInt 2; JInt 3]]
  -> '[1,true,null,"hi",[2,3]]'   length 24

Exercises:
  - five-constructor ADT (one nullary, three single-arg, one list-arg)
  - recursive match
  - String.concat ',' (List.map to_string xs)
  - string-cat with embedded escaped quotes

34 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -16,6 +16,7 @@
   "frequency.ml": 5,
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
+  "json_pretty.ml": 24,
   "lambda_calc.ml": 7,
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,