[Feature Request] Iteration index() within loops

[SolidSnakeGr]

I've just come accross this issue..

So far, we can use the key() and iteration() functions when running within loops but sometimes an index() function (return the current iteration number) would also be useful in case we would like to make something happen when the first/specified/last iteration occurs or add the iteration number in a class.

Especially when working with associative arrays or objects it's necessary to be able to retrieve the current index of the iteration.

Would that be hard to achieve?