Proposal: Add ReverseArrayIterator and ForwardArrayIterator to SPL

Hello, everyone! This proposal adds two new classes to the SPL: - Spl\ReverseArrayIterator. It iterates over an array in reverse order. It does not duplicate the array. - Spl\ForwardArrayIterator. It iterates over an array in forward (normal) order. It does not duplicate the array. They both implement Countable which returns the count() of the array. The [PR][1] has some examples and discusses why I am proposing ForwardArrayIterator when there is already ArrayIterator, the short of which is for performance. There are timing numbers in [one of the comments][2]. When it comes time to vote I may merge this into another RFC with [CachedIterable by Tyson Andre][3], which I recommend readers also take a look at. Whether we team up for the RFC vote or not, I wanted to get this out there for discussion and review. [1]: https://github.com/php/php-src/pull/6535 [2]: https://github.com/php/php-src/pull/6535#issuecomment-769179450 [3]: https://github.com/php/php-src/pull/6655