Make iterateRight sound

[LukaJCB]

Small fix for #1973. I'm not sure if the implementation is the most efficient, so feedback would be greatly appreciated! :)

Here's the code I used to test the unsoundness:

scala> Foldable.iterateRight(List(1,2,3).iterator, Eval.now(0)) { (a, eb) => 
     |   eb.flatMap(b => Eval.always(a + b))
     | }
res0: cats.Eval[Int] = cats.Eval$$anon$9@432234e6

scala> res0.value
res1: Int = 6

scala> res0.value
res2: Int = 0

[johnynek]

You’re a machine!

Thanks for fixing so quickly.

[codecov-io]

Codecov Report

Merging #1980 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1980      +/-   ##
==========================================
- Coverage   96.21%   96.21%   -0.01%     
==========================================
  Files         272      272              
  Lines        4628     4627       -1     
  Branches      124      115       -9     
==========================================
- Hits         4453     4452       -1     
  Misses        175      175

Impacted Files	Coverage Δ
core/src/main/scala/cats/Foldable.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/instances/map.scala	94.28% <100%> (-0.16%)	⬇️
core/src/main/scala/cats/instances/set.scala	100% <100%> (ø)	⬆️

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[kailuowang]

@LukaJCB sorry, merge conflicts.