Restructure `arrow` and `functor` package.

[kailuowang]

1. move arrow to a separate module.
2. Move Profunctor and Strong to this module arrow
3. Move Bifunctor, Invariant and Contravariant to core

[kailuowang]

Please vote if you, like me, think it's mostly bikeshedding. But if you have stronger feeling to put down, please do, it will be helpful for sure.

[tpolecat]

If you agree with my organizing principle for the infographic the outliers are pretty evident I think.

[kailuowang]

looks like the decision can be made. I am moving this issue to "ready"

[kailuowang]

working on a PR.

[kailuowang]

arrow.FunctionK depends on cats.core, I incline to keep it in cats.core but it means that I need to move it to the cats root package. cc @sellout. Speaking of FunctionK its documentation is in the data types section, but I am not sure if that's accurate.

[sellout]

@kailuowang Having FunctionK in core sounds good to me.

[LukaJCB]

I think adding it to data does make some sense, as we have mostly type classes in cats . Don't have any strong opinion either way though.

[kailuowang]

the part I am not sure about is if FunctionK is a data type. Anyway moving arrow to a module is more complicated than I thought, since Category and Compose depends on SemigroupK and MonoidK, so more triage needed for that one. I'll submit one PR for restructuring the functor package first.

[LukaJCB]

Well, I'd say it's as much a data type as Kleisli and cokleisli, no? I think we could put everything that's not a type class into data :D