use as and void instead of map(_ => )

[julien-truffaut]

No description provided.

[johnynek]

This is great. One problem with all these specialized methods is that people often forget them and cause potential for deoptimizations.

[LukaJCB]

Looks good 👍

[codecov-io]

Codecov Report

Merging #2403 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2403   +/-   ##
=======================================
  Coverage   95.01%   95.01%           
=======================================
  Files         349      349           
  Lines        6000     6000           
  Branches      222      224    +2     
=======================================
  Hits         5701     5701           
  Misses        299      299

Impacted Files	Coverage Δ
core/src/main/scala/cats/Functor.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/Kleisli.scala	97.08% <100%> (ø)	⬆️
core/src/main/scala/cats/FlatMap.scala	81.81% <100%> (ø)	⬆️
core/src/main/scala/cats/Reducible.scala	95.38% <100%> (ø)	⬆️

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