implement std functions from nixpkgs.lib

nib/std/lists.nix

@@ -0,0 +1,11 @@
+rec {
+  foldl = op: nul: list: let
+    foldl' = n:
+      if n == -1
+      then nul
+      else op (foldl' (n - 1)) (builtins.elemAt list n);
+  in
+    foldl' (builtins.length list - 1);
+
+  crossLists = f: foldl (fs: args: builtins.concatMap (f: map f args) fs) [f];
+}