Add traverseEither

[LukaJCB]

I find this combinator really useful, as it doesn't discard information in the way traverseFilter does.

[codecov-io]

Codecov Report

Merging #3294 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3294      +/-   ##
==========================================
+ Coverage   93.14%   93.15%   +<.01%     
==========================================
  Files         378      378              
  Lines        7575     7579       +4     
  Branches      194      203       +9     
==========================================
+ Hits         7056     7060       +4     
  Misses        519      519

Flag	Coverage Δ
#scala_version_212	93.4% <100%> (ø)	⬆️
#scala_version_213	92.93% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/TraverseFilter.scala	100% <100%> (ø)	⬆️
...ala/cats/laws/discipline/TraverseFilterTests.scala	100% <100%> (ø)	⬆️
.../src/main/scala/cats/laws/TraverseFilterLaws.scala	100% <100%> (ø)	⬆️

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[travisbrown]

Looks reasonable to me!