- previously returned the range slice as a UList,
  but this prevents convenient assignment.
  Apply similar handling for Field/SubField

  Allows the following

     labelRange range(...);
     fullList.slice(range) = identity(range.size());

  and

     fullList.slice(range) = UIndirectList<T>(other, addr);

ENH: create SubList from full FixedList (simplifies interface)

- allow default constructed SubList. Use shallowCopy to 'reset' later