Flesh out ZEND_HASH_REVERSE_FOREACH_* macros

[twose]

wonder if iterating over buckets in reverse would be more efficient for rejecting arrays that weren't list, here and in ext/json

E.g. if you had an array with thousands of elements, but a few were unset in the middle of the array, then the key of the last bucket (that wasn't unset) wouldn't be zend_hash_num_elements - 1

#4886 (comment)

@TysonAndre

I agree with you, I've been looking forward to this feature for a long time, thanks.

And It looks we need ZEND_HASH_REVERSE_FOREACH_KEY now.