Add evalScan1 operator

[ghostbuster91]

Hi,

I needed such operator today and as it wasn't trivial to write it, so I decided to commit it back to the fs2 repository.

[armanbilge]

Sorry this has been sitting for so long 😕 I am going to close/re-open the PR to kick-start CI against the latest main.

[ghostbuster91]

@armanbilge No problem, thanks for looking into it :)

[ghostbuster91]

There is some failing test but I don't think that it is connected:

fs2.concurrent.TopicSuite:
  + subscribers see all elements published 0.042s
==> X fs2.concurrent.TopicSuite.unregister subscribers under concurrent load  0.05s munit.ComparisonFailException: /home/runner/work/fs2/fs2/core/shared/src/test/scala/fs2/concurrent/TopicSuite.scala:114
113:        .map { result =>
114:          assertEquals(result.toMap.size, subs / 2)
115:          assertEquals(result.toMap, expected)
values are not the same
=> Obtained
6
=> Diff (- obtained, + expected)
-6
+5
    at munit.Assertions.failComparison(Assertions.scala:274)
    at flatMap @ fs2.Compiler$Target.flatMap(Compiler.scala:163)
    at flatMap @ fs2.Compiler$Target.flatMap(Compiler.scala:163)

[armanbilge]

Yeah I think that's the known flake ...

• TopicSuite is flaking frequently #3071

[armanbilge]

Is this repeated from evalScan? Can we extract it into a private helper evalScan_ like is used for scan/scan1 ?

[ghostbuster91]

Sounds like a good idea, let me try it.