Add scanrLazy for lazy lists

[drewolson]

No description provided.

[garyb]

Thanks!

[drewolson]

Thanks for merging @garyb. I was unsure if I should be exposing a new function named scanrLazy or be removing the re-exported scanr from Data.Traversable and implementing this as scanr.

The file seems inconsistent (e.g. filter vs. foldrLazy). Thoughts?

[garyb]

Yeah, I think it's a bit of a mess in there at the moment, perhaps next time we make a breaking release we can clean things up a bit!