Added PartialIteration class used when rendering collections

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?+

"template_name_counter" variable is kept but is deprecated.

[Joel Junström + Lucas Uyezu]
No need to have a file to PartialIteration class

This class is only used on the PartialRenderer.
Build only one PartialIteration object for loop
Document the PartialIteration object
Add CHANGELOG entry for PartialIteration.

Closes #7698.
Merge branch 'joeljunstrom-local-iterator-for-partial-collections'

Squash and merge #7698 doing some improvements to the original
implementation.