Add dual categories.

[sellout]

No description provided.

[codecov-io]

Codecov Report

Merging #2217 into master will decrease coverage by <.01%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #2217      +/-   ##
==========================================
- Coverage   95.04%   95.03%   -0.01%     
==========================================
  Files         333      334       +1     
  Lines        5789     5799      +10     
  Branches      211      222      +11     
==========================================
+ Hits         5502     5511       +9     
- Misses        287      288       +1

Impacted Files	Coverage Δ
...rc/main/scala/cats/laws/discipline/Arbitrary.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/Op.scala	87.5% <87.5%> (ø)

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[LukaJCB]

This looks great to me, thanks! One thing to consider is if we want to newtype this instead of wrapping like we did for NonEmptySet and NonEmptyMap?

[kailuowang]

@LukaJCB what do you think? shall we let this one in for Cats 1.2.0, or you think we the newtype is what we must have?

[LukaJCB]

No I think this is probably fine to go for now :)