lens type-classes instances

[yairchu]

Supporting the appropriate type-classes from lens like Seq does could make transitioning from Seq to Vector as simple as just changing the used type constructor for code using lens.

[konsumlamm]

That's a nice idea, but I don't want to add too many dependencies (and lens has a lot). Note that I already have instances for indexed-traversable (which is reexported by lens) and the other instances should be easy to implement.

I wouldn't be opposed to an rrb-vector-lens package which adds orphan instances for the lens type classes, or an optional dependency on lens (but I'm not sure how/if that works).