Move appropriate SortedSet and SortedMap instances to kernel

[travisbrown]

I'm splitting out some of the less controversial parts of #3000 so they're not blocked by the name fixes (which I still think should happen before 3.0, even if not in 2.0).

These instances for SortedMap and SortedSet clearly belong in cats-kernel, and while there's some overhead involved in moving them and maintaining bincompat, in this case I think it's hard to argue that it's not worth it—this isn't fixing typos, it's making import cats.kernel.instances.all._ do the obviously correct thing.

[codecov-io]

Codecov Report

Merging #3001 into master will decrease coverage by 0.14%.
The diff coverage is 85.48%.

@@            Coverage Diff            @@
##           master   #3001      +/-   ##
=========================================
- Coverage   93.75%   93.6%   -0.15%     
=========================================
  Files         366     368       +2     
  Lines        6935    6948      +13     
  Branches      185     188       +3     
=========================================
+ Hits         6502    6504       +2     
- Misses        433     444      +11

Impacted Files	Coverage Δ
core/src/main/scala/cats/instances/sortedMap.scala	84.74% <0%> (-12.76%)	⬇️
...ala/cats/kernel/instances/SortedSetInstances.scala	100% <100%> (ø)
...ala/cats/kernel/instances/SortedMapInstances.scala	100% <100%> (ø)
core/src/main/scala/cats/instances/sortedSet.scala	88.23% <42.85%> (-11.77%)	⬇️

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[kailuowang]

Thanks! I still hope that we could avoid the bincompat traits by waiting for the branch, but not strongly.

[LukaJCB]

Thank you, Travis.