Incorrect bracketState implementation

[ioaoue]

The current implementation of bracketState does not guarantee that:

• use will be cancelled when fiber is cancelled
• commit will be executed if fa succeeds

The following code demonstrates the problem:

import cats.effect._
import java.time.Instant
import scala.concurrent.duration._
import tofu.syntax.bracket._

object Demo extends IOApp {
  def log(text: String): IO[Unit] = IO(println(s"${Instant.now}: $text"))

  val slow: IO[Unit] = for {
    _ <- log("start")
    _ <- Timer[IO].sleep(3.seconds)
    _ <- log("finish")
  } yield ()
  
  override def run(args: List[String]): IO[ExitCode] = 
    slow.bracketState { _ => 
      log("use").as((), ())
    } { _ =>
      log("commit")
    }.timeoutTo(1.second, log("timeout"))
      .as(ExitCode.Success)
}

The output looks like this:

2021-12-09T17:18:30.869187693Z: start
2021-12-09T17:18:33.897543168Z: finish
2021-12-09T17:18:33.899531757Z: use
2021-12-09T17:18:33.900238459Z: timeout

The "finish" message is still printed although the fiber is cancelled by timeoutTo. This is because the first FG.bracket call in bracketState code makes its init block uncancellable. The "commit" message is not printed because bracketCase calls it in the use block of the first FG.bracket, but it is not guaranteed that use will be executed after successful init.

This behavior breaks bracketModify and MVar-based tofu.memo.Cached.

[Odomontois]

Yeah, that one is tricky.
This can be easily achieved with ZIO and CE3 but can be tricky in CE2 since it doesn't privide use result for the finalizer the bracket signature

My thoughts on how and when we fix this.
This PR moves tofu.concurrent.Exit to the kernel. After that that way we can use Finally[F, tofu.concurrent.Exit] as a common interface for all three. Next, we can use it in the fixed bracketState version implementing it as a single bracket call.
We need to implement this common Finally thing for all three effects. CE2 version would have some dirty Ref hack to store use results, but it should do the trick.