ocaml: phase 5.1 number_words.ml baseline (letter count of 1..19 spelled out = 106)

19-arm match returning the English word for each number 1..19, then
sum String.length:

  let number_to_words n =
    match n with
    | 1 -> 'one' | 2 -> 'two' | ... | 19 -> 'nineteen'
    | _ -> ''

  total_letters 19 = 36 + 70 = 106
                    (1-9)  (10-19)

Real PE17 covers 1..1000 (answer 21124) but needs more elaborate
number-to-words logic (compounds, 'and', 'thousand'). 1..19 keeps
the program small while exercising literal-pattern match dispatch
on many arms.

128 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -86,6 +86,7 @@
   "module_use.ml": 3,
   "monotonic.ml": 4,
   "newton_sqrt.ml": 1414,
+  "number_words.ml": 106,
   "mutable_record.ml": 10,
   "option_match.ml": 5,
   "palindrome.ml": 4,

lib/ocaml/baseline/number_words.ml

@@ -0,0 +1,19 @@
+let number_to_words n =
+  match n with
+  | 1 -> "one" | 2 -> "two" | 3 -> "three" | 4 -> "four" | 5 -> "five"
+  | 6 -> "six" | 7 -> "seven" | 8 -> "eight" | 9 -> "nine"
+  | 10 -> "ten" | 11 -> "eleven" | 12 -> "twelve"
+  | 13 -> "thirteen" | 14 -> "fourteen" | 15 -> "fifteen"
+  | 16 -> "sixteen" | 17 -> "seventeen" | 18 -> "eighteen" | 19 -> "nineteen"
+  | _ -> ""
+
+let total_letters limit =
+  let total = ref 0 in
+  for i = 1 to limit do
+    total := !total + String.length (number_to_words i)
+  done;
+  !total
+
+;;
+
+total_letters 19