ocaml: phase 6 List.sort + compare (+7 tests, 339 total)

compare is a host builtin returning -1/0/1 (Stdlib.compare semantics)
deferred to host SX </>. List.sort is insertion-sort in OCaml: O(n²)
but works correctly. List.stable_sort = sort.

Tested: ascending int sort, descending via custom comparator (b - a),
empty list, string sort.

lib/ocaml/runtime.sx

@@ -143,6 +143,20 @@
        match lst with
        | [] -> None
        | (k2, v) :: t -> if k = k2 then Some v else assoc_opt k t
+
+     let rec sort cmp xs =
+       begin
+         let rec insert x ys =
+           match ys with
+           | [] -> [x]
+           | h :: t -> if cmp x h <= 0 then x :: ys else h :: insert x t
+         in
+         match xs with
+         | [] -> []
+         | h :: t -> insert h (sort cmp t)
+       end
+
+     let stable_sort = sort
    end ;;
 
    module Option = struct