CommutativeMonad and CommutativeFlatMap instances for Tuple2

[ceedubs]

I think that these are fairly straightforward/legitimate. Speak up if
that's not true :)

[ceedubs]

It should be possible to do the same sort of thing for Writer, but Writer is a little different in that there are a couple of different FlatMap instances defined in a hierarchy. I don't have the time to look into it right at the moment, but someone else can feel free.

[ceedubs]

I need to add some Mima exceptions. I'll do that now.

[codecov-io]

Codecov Report

Merging #2099 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2099      +/-   ##
==========================================
+ Coverage   94.65%   94.75%   +0.09%     
==========================================
  Files         322      322              
  Lines        5449     5454       +5     
  Branches      215      214       -1     
==========================================
+ Hits         5158     5168      +10     
+ Misses        291      286       -5

Impacted Files	Coverage Δ
core/src/main/scala/cats/instances/tuple.scala	100% <100%> (ø)	⬆️
...ws/src/main/scala/cats/laws/DistributiveLaws.scala	100% <0%> (ø)	⬆️
...scala/cats/laws/discipline/DistributiveTests.scala	100% <0%> (ø)	⬆️
.../main/scala/cats/laws/CommutativeFlatMapLaws.scala	100% <0%> (+33.33%)	⬆️
...cats/laws/discipline/CommutativeFlatMapTests.scala	100% <0%> (+50%)	⬆️

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[LukaJCB]

Yup these are great, thanks! :)