ENH: adjustments for ListOps subset, subsetList

- now takes a const UList<T> as input and returns a List<T>
  instead of trying to use the same ListType for both.

  This avoids previously encountered issues when a UList was passed in.

- add specialized bitSet handling within subset(), where we can benefit
  from faster traversal of sparse selections and have a better estimate
  of the final output size.