Implement LazyList tailRecM using Iterator.unfold

[andyscott]

Implements LazyList's tailRecM using Iterator.unfold.

[codecov-io]

Codecov Report

Merging #2964 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2964      +/-   ##
==========================================
+ Coverage   93.91%   93.93%   +0.02%     
==========================================
  Files         371      371              
  Lines        7061     7061              
  Branches      177      177              
==========================================
+ Hits         6631     6633       +2     
+ Misses        430      428       -2

Impacted Files	Coverage Δ
...cala/cats/kernel/instances/FunctionInstances.scala	100% <0%> (+3.03%)	⬆️
testkit/src/main/scala/cats/tests/Helpers.scala	100% <0%> (+4%)	⬆️

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[codecov-io]

Codecov Report

Merging #2964 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2964      +/-   ##
==========================================
+ Coverage   93.91%   93.93%   +0.02%     
==========================================
  Files         371      371              
  Lines        7061     7061              
  Branches      177      177              
==========================================
+ Hits         6631     6633       +2     
+ Misses        430      428       -2

Impacted Files	Coverage Δ
...cala/cats/kernel/instances/FunctionInstances.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 3f3c979...099b7a7. Read the comment docs.

[kailuowang]

cool! thanks!

[johnynek]

two small notes, I bet this would be faster if we did:

unfold[Option[B], List[Iterator[Either[A, B]]])(Iterator.single(Left(a)) :: Nil) {
  case Nil => None
  case nonempty@(h :: t) =>
    if (!h.hasNext) Some((None, t))
    else h.next match {
      case Left(a) => Some((None, fn(a) :: nonempty))
      case Right(b) => Some((Some(b), nonempty))
    } 
}