added index to rpn item for location within output list

this index will be needed by the encoder to determine the location of
attached items; access functions were added for the index including an
increment function needed to adjusting indexes when items are inserted
into the output list

updated outputAppend() to assign index when item appended to list
updated outputInsert() to adjust indexes of items after insert point

modified text output of RpnItem to include index of attached items;
updated expected results that were affected