Make kittens officially a Cats module (still separate repo)

[kailuowang]

I had a discussion with @milessabin see (typelevel/kittens#96). We think that making kittens an official cats module (still in its own separate repo) would be a good idea. @milessabin moved it to under github typelevel. I'm proposing this to the Cats community through this PR.

Update: I updated the title to further clarify the intention.

[ceedubs]

I'm a bit hesitant on this. Kittens depends on Shapeless, right? Here are some concerns that I would have:

• The Cats builds are already cross-building against several Scala versions and Scala.js. I think that them potentially needing to cross-compile against multiple Shapeless versions someday would be rough (in a combinatorial way).
• Because of its more complex usage of the type system and heavier usage of macros, I could see Shapeless being quicker to drop older versions of Scala. This could get tricky to try to not cross-build just the kittens module for some scala versions.

I'm happy to see kittens in the typelevel org, but I question the cost/benefit ratio of making it a module within the cats repo.

[kailuowang]

@ceedubs not a module inside cats repo, it's still in its own repo with its own release cycle just like the other 3 "outside" modules cats-mtl, cats-effect and mouse.

[tpolecat]

Does "official cats module" mean anything other than appearing on this bullet list? This is fine, it's just probably worth clarifying.

[kailuowang]

@tpolecat that's mostly it. Although I'd say that cats modules get a commitment from Cats maintainers to help to maintain them.

[ceedubs]

@kailuowang ah okay. I understand. I'm all for this then.

I do have a concern about kittens derivation that has prevented me from using it and I've been meaning to look for a discussion of, but I'll pursue that separately.

[LukaJCB]

Regarding commitment to maintain, I'd happily help out, but I fear my shapeless skills really aren't up to par. I do think kittens is an important project that we (heh) should give more focus, so I really like this move :)

[codecov-io]

Codecov Report

Merging #2361 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2361   +/-   ##
=======================================
  Coverage   95.01%   95.01%           
=======================================
  Files         349      349           
  Lines        5993     5993           
  Branches      223      225    +2     
=======================================
  Hits         5694     5694           
  Misses        299      299

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[kailuowang]

seems that we have enough approval. It's official!