rpn list: change list from base class to member

having the underlying list as a base class turned out to be problematic
downstream, so the list was made a member of the class instead with
needed access functions added

the associated translator list access functions were insured to use the
rpn list class access functions instead of the list directly (only the
output last token function needed to be modified)

the call to clear the list in the destructor was removed since this is
what the destructor does anyway