function repeat(coll, n) {
  if (arguments.length === 1) {
    n = coll;
    return function(xform) {
      return new Repeat(n, xform);
    };
  }
  return seq(coll, repeat(n));
}
Returns a new collection containing elements of the given
collection, each repeated n times. Returns a transducer if a
collection is not provided.