Fix all compiler warnings on Scala 2.13

[kailuowang]

Fatal-warning was temporarily disabled on Scala 2.13. Removing them might require breaking changes, hence we'd better do it before 2.0-RC1
This might be too much as a single task. We should probably achieve this with multiple PRs.
If anyone is interested, please feel free to choose a smaller scope to work on and announce it here. So that we can parallelize.

[poslegm]

Should cats 2.0 for Scala 2.13 contains instances for Stream? It's deprecated and replaced by LazyList.

[kailuowang]

@poslegm thanks for looking at this. I would say no, cats 2.0 code base should not have any deprecation warnings. Thus we need to move all the Stream instances to the scala version specific source folders /core/src/main/scala-2.12-/cats/. and probably need a blank StreamInstances trait in the 2.13+ folder For the first PR I don't think we have to add the LazyList instance yet, that can be a different PR.

Stream is also used in laws/ExhausiveCheck, in which case maintaining cross build might be hard. I propose we replace that with a List which was discussed during the PR introducing ExhausiveCheck. We ended didn't switch to List at that time due to lack of benefit but now we do. That could be a separate PR if you want to control the scope of the one you are working on.

Update: I ended up creating the PR for replacing Stream in ExhausiveCheck. Also added Scala version specific folders for tests in the build. It is necessary when we move all the Stream instances and related tests to version specific folders
#2896

Thanks!!!

[kailuowang]

@poslegm I realized that we probably still want to keep Stream instances in 2.13 but deprecate them only on 2.13, this will require duplication the code in two scala version specific folders. I think I am going to take stab myself.

[kailuowang]

Update Stream deprecations are resolved by #2904