Add flatTransform to OptionT (Issue #2309)

[sh-abhi]

As suggested by @amilkov3 and @kubukoz.

Referencing #2309

[codecov-io]

Codecov Report

Merging #2313 into master will increase coverage by 0.29%.
The diff coverage is 94.93%.

@@            Coverage Diff             @@
##           master    #2313      +/-   ##
==========================================
+ Coverage   94.73%   95.03%   +0.29%     
==========================================
  Files         338      338              
  Lines        5868     5879      +11     
  Branches      213      212       -1     
==========================================
+ Hits         5559     5587      +28     
+ Misses        309      292      -17

Impacted Files	Coverage Δ
core/src/main/scala/cats/Foldable.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/Bitraverse.scala	80% <ø> (ø)	⬆️
core/src/main/scala/cats/data/EitherT.scala	97.7% <100%> (ø)	⬆️
core/src/main/scala/cats/syntax/binested.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/Binested.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/syntax/parallel.scala	72.72% <100%> (ø)	⬆️
core/src/main/scala/cats/syntax/set.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptySet.scala	98.75% <100%> (ø)	⬆️
laws/src/main/scala/cats/laws/discipline/Eq.scala	94.33% <100%> (ø)	⬆️
.../src/main/scala/cats/syntax/applicativeError.scala	100% <100%> (ø)	⬆️
... and 21 more

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[kubukoz]

Looks like you got some changes unrelated to the issue here... Including #2299, #2285 etc. - can you create a new branch that only has your changes, and create a new PR?

[johnynek]

+1 to separate PRs here. Each can be reviewed and merged on their own schedule.

[johnynek]

On second look, this looks like some GitHub issue. Can you rebase on master?

[sh-abhi]

@kubukoz @johnynek Closing this and opened a new one: #2314.