add traverseN to cartesian syntax

[julien-truffaut]

No description provided.

[kailuowang]

Shall we change title to "add traverseN to cartesian syntax", which is what it does. CartesianBuilder is deprecated.

[julien-truffaut]

I am not sure what does it mean:

[error] /home/travis/build/typelevel/cats/tests/src/test/scala/cats/tests/SyntaxTests.scala:147: discarded non-Unit value
[error]     (fa, fb, fc) traverseN ft

[kailuowang]

Either add a () at the end of the method or make it a statement by adding a val blah = (fa, fb, fc) traverseN ft which is a ()

[codecov-io]

Codecov Report

Merging #1942 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1942      +/-   ##
==========================================
+ Coverage   95.57%   95.61%   +0.04%     
==========================================
  Files         248      248              
  Lines        4454     4519      +65     
  Branches      117      120       +3     
==========================================
+ Hits         4257     4321      +64     
- Misses        197      198       +1

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/EitherT.scala	98.36% <0%> (ø)	⬆️
core/src/main/scala/cats/syntax/alternative.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/Alternative.scala	90.9% <0%> (+2.02%)	⬆️

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