ocaml: phase 6 List.take/drop/filter_map/flat_map (+6 tests, 400 total)

Common functional helpers written in OCaml. flat_map / concat_map
share an implementation. 400-test milestone.

lib/ocaml/runtime.sx

@@ -198,6 +198,35 @@
          match ys with
          | [] -> failwith \"List.map2: unequal\"
          | hy :: ty -> f hx hy :: map2 f tx ty
+
+     let rec take n xs =
+       if n <= 0 then []
+       else
+         match xs with
+         | [] -> []
+         | h :: t -> h :: take (n - 1) t
+
+     let rec drop n xs =
+       if n <= 0 then xs
+       else
+         match xs with
+         | [] -> []
+         | _ :: t -> drop (n - 1) t
+
+     let rec filter_map f xs =
+       match xs with
+       | [] -> []
+       | h :: t ->
+         match f h with
+         | None -> filter_map f t
+         | Some v -> v :: filter_map f t
+
+     let rec flat_map f xs =
+       match xs with
+       | [] -> []
+       | h :: t -> append (f h) (flat_map f t)
+
+     let concat_map = flat_map
    end ;;
 
    module Option = struct