lib/lists: mutuallyExclusive function optimization

The current implementation of `mutuallyExclusive` builds a new list with
length subtracted by one on every recursive call which is expensive. When
b is empty, the function still traverses a in its entirety before returning
a result.

The new implementation uses `any` to check if each element of list b is in
list a using `elem`. This maintains short circuiting when list a or b is empty
and has a worst case time complexity of O(nm).

lib/lists.nix

@@ -663,9 +663,6 @@ rec {
   /* Test if two lists have no common element.
      It should be slightly more efficient than (intersectLists a b == [])
   */
-  mutuallyExclusive = a: b:
-    (builtins.length a) == 0 ||
-    (!(builtins.elem (builtins.head a) b) &&
-     mutuallyExclusive (builtins.tail a) b);
+  mutuallyExclusive = a: b: length a == 0 || !(any (x: elem x a) b);
 
 }