- 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
