Add Either.unit, use in Applicative[Either[A, ?]]

[rossabaker]

Fixes #3343. Thanks to @sirthias for the idea.

[codecov-io]

Codecov Report

Merging #3346 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3346      +/-   ##
==========================================
+ Coverage   93.32%   93.33%   +<.01%     
==========================================
  Files         378      378              
  Lines        7689     7692       +3     
  Branches      206      203       -3     
==========================================
+ Hits         7176     7179       +3     
  Misses        513      513

Flag	Coverage Δ
#scala_version_212	93.38% <100%> (+0.02%)	⬆️
#scala_version_213	93.11% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/instances/either.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/syntax/either.scala	83.33% <100%> (+0.24%)	⬆️

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