Support for Cats 0.8.0 and beyond

[od0]

With version 0.8.0, Cats sunsetted Xor. This raises conflicts when using Roc alongside libraries that have migrated to Cats 0.8.0+ (e.g. https://github.com/finagle/finch)

There are only a handful of occurrences to convert to Either - I can submit a PR if that would help

[penland365]

@od0 That would be a tremendous help. I was going to look at this over the holidays, but if you've got the bandwidth I'd love a PR.

This needs to happen sooner rather than later for sure.

[od0]

@penland365 I think I may have volunteered a bit too quickly :) getting the Xor's migrated to Either is a bit beyond my bandwidth at the moment. Might be better to tackle it with a big version upgrade instead - e.g. Circe (0.5.0-M2 => 0.6.1 or 0.7 when it's ready) and Finagle (6.38.0 => 6.41.0). The latest Finagle release was a big one as it unlocks supporting Scala 2.12 (where Either is right-biased like Xor) - one more thing for what is seeming like a bigger PR :)

[od0]

@penland365 I took a quick crack and resolved most of the Xor => Either issues, but there's still compile errors which I wasn't able to immediately resolve. I opened a PR which you can contribute too as well - hope that helps in some small way :)