added a default id for Arrow

[kailuowang]

we already have a law

    F.lift(identity[A]) <-> F.id[A]

so it's hard to see why this shouldn't be given as a default.

[ceedubs]

LGTM assuming build passes

[codecov-io]

Codecov Report

Merging #2072 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2072      +/-   ##
==========================================
- Coverage   94.66%   94.66%   -0.01%     
==========================================
  Files         318      318              
  Lines        5383     5419      +36     
  Branches      207      208       +1     
==========================================
+ Hits         5096     5130      +34     
- Misses        287      289       +2

Impacted Files	Coverage Δ
core/src/main/scala/cats/instances/function.scala	92.3% <ø> (ø)	⬆️
core/src/main/scala/cats/data/Cokleisli.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/data/Kleisli.scala	97.7% <100%> (ø)	⬆️
core/src/main/scala/cats/arrow/Arrow.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/Invariant.scala	94.36% <0%> (-0.88%)	⬇️
core/src/main/scala/cats/Alternative.scala	93.75% <0%> (+4.86%)	⬆️

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