ocaml: phase 5.1 trie.ml baseline (prefix tree, 6/9 word lookups match)

Mutable-record trie with linked-list children:

  type trie = {
    mutable terminal : bool;
    mutable children : (char * trie) list
  }

Insert {cat, car, card, cart, dog, doge}; lookup 9 words. Hits are
exactly the inserted set: cat, car, card, cart, dog, doge = 6.
Misses: ca (prefix not terminal), dogs (extends 'dog' but no 'dogs'
node), x (no path).

Tests:
  - recursive type definition with self-referential field
  - mutable record fields with .field <- v
  - Option pattern matching (Some / None)
  - tuple-cons pattern (k, v) :: rest

146 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -140,6 +140,7 @@
   "sum_squares.ml": 385,
   "tree_depth.ml": 4,
   "triangle.ml": 11,
+  "trie.ml": 6,
   "triangle_div.ml": 120,
   "twosum.ml": 5,
   "union_find.ml": 4,