ocaml: phase 5.1 huffman.ml baseline (Huffman tree WPL = 224)

Classic CLRS Huffman code example. ADT:

  type tree = Leaf of int * char | Node of int * tree * tree

Build by repeatedly merging two lightest trees (sorted-list pq):

  let rec build_tree lst = match lst with
    | [t] -> t
    | a :: b :: rest ->
      let merged = Node (weight a + weight b, a, b) in
      build_tree (insert merged rest)

  weighted path length (= total Huffman bits):
    leaves {(5,a) (9,b) (12,c) (13,d) (16,e) (45,f)} -> 224

Tests sum-typed ADT with mixed arities, `function` keyword
pattern matching, recursive sorted insert, depth-counting recursion.

150 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -65,6 +65,7 @@
   "hamming.ml": 4,
   "hanoi.ml": 1023,
   "hist.ml": 75,
+  "huffman.ml": 224,
   "int_sqrt.ml": 1027,
   "is_prime.ml": 25,
   "fizz_classifier.ml": 540,