ocaml: phase 5.1 mst_kruskal.ml baseline (5-node MST weight 11)

Kruskal's minimum spanning tree using path-compressing union-find:

  edges (w, u, v):
    (1, 0, 1) (2, 1, 2) (3, 0, 3) (4, 2, 3) (5, 3, 4) (6, 0, 4)

After sorting by weight and greedily unioning:
  pick (1,0,1) -> components: {0,1} {2} {3} {4}
  pick (2,1,2) -> {0,1,2} {3} {4}
  pick (3,0,3) -> {0,1,2,3} {4}
  skip (4,2,3) -- already connected
  pick (5,3,4) -> {0,1,2,3,4}
  skip (6,0,4) -- already connected

  MST weight = 1 + 2 + 3 + 5 = 11

Tests List.sort with 3-tuple destructuring lambda, compare on int,
Array.init with closure, in-place array mutation in find, boolean
union returning true iff merge happened.

147 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -89,6 +89,7 @@
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
   "mortgage.ml": 1073,
+  "mst_kruskal.ml": 11,
   "merge_intervals.ml": 12,
   "merge_sort.ml": 44,
   "merge_two.ml": 441,