Add lens, traversals and indexed traversals

[infinity0]

It would be nice to have lens instances so people using lens with Data.Map etc can easily migrate to this package.

[dmwit]

I'm open to this, but I don't have enough lens experience to know which instances are the right ones to write. Feel free to send me a pull request, though!

There's also the concern that lens is a pretty heavyweight dependency for what is otherwise a pretty light package. We could perhaps have separate but canonical packages with orphan instances, similar to how the universe-* family of packages work?

[infinity0]

Yes having orphan instances in a separate package such as ordered-containers-lens is a pretty common pattern for lens. As for writing them yourself, there are examples for the standard containers Data.Map etc in the lens package, you could hopefully just re-use them with minimal adjustments.