Added PartialIteration object used when rendering collections

Based on the ideas in #5634 but refactored into an object as per the comments.

The iteration object is available as the local variable
"template_name_iteration" when rendering partials with collections.

It gives access to the +size+ of the collection beeing iterated over,
the current +index+ and two convinicence methods +first?+ and +last?+

For now the "template_name_counter" variable is kept but is deprecated.