Add leftFlatMap to Either

[rohanshah]

Motivation

Either should have leftFlatMap for cases when you want to leftMap a function that returns an Either and flatten the total result.

[codecov-io]

Codecov Report

Merging #2149 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2149      +/-   ##
==========================================
+ Coverage   94.66%   94.67%   +<.01%     
==========================================
  Files         328      328              
  Lines        5533     5536       +3     
  Branches      199      199              
==========================================
+ Hits         5238     5241       +3     
  Misses        295      295

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/either.scala	99.14% <100%> (+0.02%)	⬆️

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[kailuowang]

would love to see some tests

[rohanshah]

@kailuowang yeah, I was wondering where to put them. The only test file I found for Either was EitherSuite.scala but that didn't even have a test for flatMap in it.

[kailuowang]

EitherSuite should be the right place. My guess is that flatMap isn't explicitly tested there because it's covered by the law tests

[rohanshah]

@kailuowang added tests!