Finish adding mapK

[LukaJCB]

Should finish #1987, so we can release RC1 with it! Thanks a lot @andyscott! 😊

[kailuowang]

Since the original PR already got 2 sign-offs I am going to merge this one once build is green.

[codecov-io]

Codecov Report

Merging #2000 into master will decrease coverage by 0.18%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2000      +/-   ##
==========================================
- Coverage   95.26%   95.08%   -0.19%     
==========================================
  Files         301      301              
  Lines        4921     4941      +20     
  Branches      126      123       -3     
==========================================
+ Hits         4688     4698      +10     
- Misses        233      243      +10

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Func.scala	96.42% <0%> (-3.58%)	⬇️
core/src/main/scala/cats/data/Tuple2K.scala	94.44% <0%> (-2.7%)	⬇️
core/src/main/scala/cats/data/EitherK.scala	97.82% <0%> (-2.18%)	⬇️
free/src/main/scala/cats/free/Yoneda.scala	80% <0%> (-20%)	⬇️
free/src/main/scala/cats/free/Coyoneda.scala	84.61% <0%> (-7.06%)	⬇️
core/src/main/scala/cats/data/Nested.scala	94.82% <0%> (-1.67%)	⬇️
core/src/main/scala/cats/data/OneAnd.scala	96.77% <0%> (-1.59%)	⬇️
core/src/main/scala/cats/data/IndexedStateT.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/EitherT.scala	97.58% <100%> (+0.01%)	⬆️
...in/scala/cats/data/IndexedReaderWriterStateT.scala	100% <100%> (ø)	⬆️
... and 6 more

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