Derive Macro Iterators

#[derive(Iterators)]

A procedural macro fully implementing [IterableByValue] and [IterableByValueFrom] for subslices on top of a the SubsliceImpl structure generated by the derive macro [Subslice].

The macro defines a structure Iter that keeps track of a mutable reference to a slice, and of a current position, and that is used to implement IterableByValue on SubsliceImpl.