convert trait into abstract class for better bin compact

[kailuowang]

related #1879

[codecov-io]

Codecov Report

Merging #1882 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1882   +/-   ##
=======================================
  Coverage   94.97%   94.97%           
=======================================
  Files         241      241           
  Lines        4199     4199           
  Branches      106      106           
=======================================
  Hits         3988     3988           
  Misses        211      211

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/NonEmptyList.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/Eval.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/data/NonEmptyVector.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/data/OneAnd.scala	98.36% <ø> (ø)	⬆️
core/src/main/scala/cats/data/OptionT.scala	100% <ø> (ø)	⬆️
free/src/main/scala/cats/free/FreeT.scala	91.66% <ø> (ø)	⬆️

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[ceedubs]

👍 LGTM

[ceedubs]

Merging since this is a minor documentation fix.

[ceedubs]

Oops this isn't the PR that I meant to merge 😬. I think that it's probably fine though. If anyone has any concerns, please let me know.

[kailuowang]

I agree it should be fine.