Decide on a solution for adding new features to cats-kernel avoiding breaking bin-compat.

[kailuowang]

related #1878 #1712 #1527

[kailuowang]

Also we'll probably quickly run into the same question IRT cats-core after 1.0.0. So hopefully the solution we come up with kernel will be a good reference.

[johnynek]

1 think we could do is try to be more aggressive about using abstract class over trait which is kinder on binary compatibility (although that may not be true in scala 2.12 since I think trait encoding changed).

Aside from that, scala is pretty sensitive to binary change (one day, we will live in a source dependency world, I hope...). And we want cats to be at the bottom of a lot of dep graphs, we need a high degree of compatibility.

[kailuowang]

good point @johnynek , I am creating a PR to change existing "instance" trait to abstract class.

[kailuowang]

removed from RC1 scope, since all breaking changes are okayed by @johnynek representing the Algebird community.