ocaml: phase 5.1 dijkstra.ml baseline (5-node SSSP, dist(0,4) = 7)

Array-based O(n^2) Dijkstra on a small directed weighted graph:

  edges = [|
    [(1, 4); (2, 1)];   (* 0 -> 1 (w=4), 2 (w=1) *)
    [(3, 1)];           (* 1 -> 3 (w=1)         *)
    [(1, 2); (3, 5)];   (* 2 -> 1 (w=2), 3 (w=5) *)
    [(4, 3)];           (* 3 -> 4 (w=3)         *)
    []                  (* 4 sink              *)
  |]

Optimal path 0->2->1->3->4 has weight 1+2+1+3 = 7.

Tests: array-of-list-of-int-pair literal, List.iter with tuple
destructuring closure, in-place dist mutation, nested for + ref.

141 baseline programs total.

lib/ocaml/baseline/expected.json

@@ -25,6 +25,7 @@
   "coin_change.ml": 6,
   "count_change.ml": 406,
   "csv.ml": 10,
+  "dijkstra.ml": 7,
   "exception_handle.ml": 4,
   "exception_user.ml": 26,
   "euler1.ml": 233168,