Define OptionT.mapFilter in terms of OptionT.subflatMap

[morgen-peschke]

fixes #2757

[kailuowang]

Thanks!

[codecov-io]

Codecov Report

Merging #2765 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2765      +/-   ##
==========================================
+ Coverage   95.31%   95.34%   +0.02%     
==========================================
  Files         363      363              
  Lines        6785     6785              
  Branches      285      283       -2     
==========================================
+ Hits         6467     6469       +2     
+ Misses        318      316       -2

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/OptionT.scala	96.29% <100%> (ø)	⬆️
...rc/main/scala/cats/kernel/instances/function.scala	100% <0%> (+3.03%)	⬆️
testkit/src/main/scala/cats/tests/Helpers.scala	100% <0%> (+4%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 56a1339...3f19495. Read the comment docs.