Add choice syntax

[catostrophe]

Choice doesn't have a separate syntax object in cats.syntax package and its syntax (choice, |||) is not accessible via import cats.implicits._

[codecov-io]

Codecov Report

Merging #2315 into master will increase coverage by 0.31%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2315      +/-   ##
==========================================
+ Coverage   94.73%   95.04%   +0.31%     
==========================================
  Files         338      338              
  Lines        5868     5878      +10     
  Branches      213      207       -6     
==========================================
+ Hits         5559     5587      +28     
+ Misses        309      291      -18

Impacted Files	Coverage Δ
core/src/main/scala/cats/arrow/Choice.scala	100% <ø> (+50%)	⬆️
laws/src/main/scala/cats/laws/ChoiceLaws.scala	100% <100%> (ø)	⬆️
laws/src/main/scala/cats/laws/MonadLaws.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/data/Ior.scala	98.48% <0%> (+0.01%)	⬆️
core/src/main/scala/cats/Eval.scala	98.75% <0%> (+0.04%)	⬆️
core/src/main/scala/cats/data/IorT.scala	97.76% <0%> (+0.05%)	⬆️
core/src/main/scala/cats/syntax/either.scala	99.16% <0%> (+14.16%)	⬆️

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[kailuowang]

Thanks @catostrophe

[LukaJCB]

LGTM :)