lib/util: add groupListBySize

Splits up a list into many sub-lists based on the given max-size.

e.g.
```nix
groupListBySize 2 [ 1 2 3 4 5 ]
=> [ [ 1 2 ] [ 3 4 ] [ 5 ] ]
```

lib/utils.nix

@@ -147,4 +147,23 @@ rec {
       ${string}
       EOF
     '';
+
+  # Split a list into a several sub-list, each with a max-size of `size`
+  groupListBySize =
+    size: list:
+    reverseList (
+      foldl' (
+        lists: item:
+        let
+          first = head lists;
+          rest = drop 1 lists;
+        in
+        if lists == [ ] then
+          [ [ item ] ]
+        else if length first < size then
+          [ (first ++ [ item ]) ] ++ rest
+        else
+          [ [ item ] ] ++ lists
+      ) [ ] list
+    );
 }