reduce constraint of foldMapM from Monad to Applicative

[julien-truffaut]

fix #2852

I am not sure if it is binary compatible since Applicative extends Monad and it is in a Contravariant position.

[codecov-io]

Codecov Report

Merging #2854 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2854   +/-   ##
=======================================
  Coverage   94.21%   94.21%           
=======================================
  Files         368      368           
  Lines        6944     6944           
  Branches      301      308    +7     
=======================================
  Hits         6542     6542           
  Misses        402      402

Impacted Files	Coverage Δ
core/src/main/scala/cats/Foldable.scala	100% <100%> (ø)	⬆️

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[kailuowang]

This is not BC, once we drop Scala 2.11 on master, we can add a foldMapA for this if it helps.

[julien-truffaut]

@kailuowang yeah that's what I expected but I wasn't sure. Should I revert my last commit and add foldMapA? We can merge this whenever 2.11 is dropped

[kailuowang]

Yeah, let's do that. It will be a bit more work though since you need to add tests.

[LukaJCB]

Shall we add foldA at that point as well? :)

[julien-truffaut]

I don't have the bandwidth to finish the work, closing for now