Functional programming support for iterable classes. The class must implement the iterator and newInstance methods.

For most methods, the iterator only needs to implement next and atEnd. Backwards iterators like reduceRight require prev and atBeginning.

Iterable classes must mix these functions: `Y.mix(SomeClass, Y.Iterable, false, null, 4);` Passing false as the third argument allows your class to provide optimized implementations of individual functions.