ocaml: phase 5.1 combinations.ml baseline (C(9, 4) = 126)

Pascal-recursion combination enumerator:

  let rec choose k xs =
    if k = 0 then [[]]
    else match xs with
      | [] -> []
      | h :: rest ->
        List.map (fun c -> h :: c) (choose (k - 1) rest)
        @ choose k rest

  C(9, 4) = |choose 4 [1; ...; 9]| = 126

Tests pure-functional enumeration with List.map + closure over h,
@ append, [] | h :: rest pattern match on shrinking input.

200 baseline programs total -- milestone.

lib/ocaml/baseline/expected.json

@@ -29,6 +29,7 @@
   "calc.ml": 13,
   "catalan.ml": 42,
   "closures.ml": 315,
+  "combinations.ml": 126,
   "convex_hull.ml": 5,
   "coin_change.ml": 6,
   "coin_min.ml": 6,