ocaml: phase 5.1 levenshtein.ml baseline (no-memo edit distance, sum=11)

Recursive Levenshtein edit distance with no memoization (the test
strings are short enough for the exponential-without-memo version to
fit in <2 minutes on contended hosts). Sums distances for five short
pairs:

  ('abc','abx') + ('ab','ba') + ('abc','axyc') + ('','abcd') + ('ab','')
   = 1 + 2 + 2 + 4 + 2 = 11

Exercises:
  * curried four-arg recursion
  * s.[i] equality test (char comparison)
  * min nested twice for the three-way recurrence
  * mixed empty-string base cases

lib/ocaml/baseline/expected.json

@@ -11,6 +11,7 @@
   "fizzbuzz.ml": 57,
   "list_ops.ml": 30,
   "lambda_calc.ml": 7,
+  "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
   "module_use.ml": 3,
   "mutable_record.ml": 10,