Indexed foldable-traversable instances

[matthewleon]

Addresses #21

[paf31]

Thank you for the PR, but it's not clear to me that Semiring is always the right choice here. Certainly, to be compatible with the non-indexed instances, it needs to be the "first" index somehow, but I don't know if that means zero necessarily. Nothing says the indices even have to come in increasing order. That said, we don't really have any laws which tell us what the index should mean in these instances.

[matthewleon]

Good points. Didn't think of this.

[matthewleon]

Does this imply that #21 should actually be closed? It would appear that there is no sensible way to do what it's asking.

[paf31]

I think so, thanks!

[safareli]

what about

mapWithIndex f (a :| fa) = f Nothing a :| mapWithIndex (Just >>> f) fa 

[paf31]

Good idea, that seems much better.

[matthewleon]

@safareli awesome. I've modified these accordingly.

[safareli]

Maybe is algebraic "+ 1", and that's what we need to do with the index type.

Welcome and thanks for making this pr @matthewleon!

[paf31]

Thanks!