Add findM to Foldable

[KentShikama]

No description provided.

[codecov-io]

Codecov Report

Merging #2365 into master will decrease coverage by 0.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2365      +/-   ##
==========================================
- Coverage   95.01%   94.73%   -0.28%     
==========================================
  Files         349      349              
  Lines        5997     5982      -15     
  Branches      223      215       -8     
==========================================
- Hits         5698     5667      -31     
- Misses        299      315      +16

Impacted Files	Coverage Δ
core/src/main/scala/cats/Foldable.scala	100% <100%> (ø)	⬆️
testkit/src/main/scala/cats/tests/CatsSuite.scala	33.33% <0%> (-36.67%)	⬇️
core/src/main/scala/cats/syntax/either.scala	85% <0%> (-14.17%)	⬇️
.../src/main/scala/cats/data/RepresentableStore.scala	83.33% <0%> (-4.17%)	⬇️
core/src/main/scala/cats/data/IorT.scala	97.74% <0%> (-0.05%)	⬇️
core/src/main/scala/cats/Eval.scala	98.79% <0%> (-0.05%)	⬇️
core/src/main/scala/cats/data/Ior.scala	98.47% <0%> (-0.02%)	⬇️
laws/src/main/scala/cats/laws/MonadLaws.scala	100% <0%> (ø)	⬆️

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[johnynek]

We can’t add this to Foldable without breaking binary compatibility very seriously.

We only break package private and sealed binary compatibility since no one using scala should have subclassed those.

[johnynek]

We could put this on a binary compatible shim as we have done in other cases.

[LukaJCB]

As @johnynek said, please add this as a syntax extension instead of a method on a type class :)

We can add these to the type class when we next break binary compatibility, i.e. 2.0.

[KentShikama]

Closing for #2421