function takeNth(coll, nth) {
  if (arguments.length === 1) {
    nth = coll;
    return function(xform) {
      return new TakeNth(nth, xform);
    };
  }
  return seq(coll, takeNth(nth));
}
Returns a new collection of every nth element of the given
collection. Returns a transducer if a collection is not provided.