Updated the code back-end to produce the same positions as ocamlyacc and the table back-end, in principle.
This involves:
1- moving the [endp] field (when it exists) to offset 1 in every stack cell,
   so it can be found when reducing an epsilon production;
2- creating a sentinel cell with an [endp] field at offset 1;
3- changing the code for epsilon productions to read the [endp] field in the
   top stack cell.