Add sleep to Pull

[nikiforo]

Added Pull.sleep, analogously to Stream.sleep.

import cats.effect.IOApp
import cats.effect.{ExitCode, IO}
import scala.concurrent.duration._
import cats.syntax.all._

object PullSleepApp extends IOApp {

  def run(args: List[String]): IO[ExitCode] =
    printTime
      .productR(Pull.sleep[IO](1.second))
      .productR(printTime)
      .stream
      .compile
      .drain
      .as(ExitCode.Success)

  private def printTime = Pull.eval(IO.delay(println(System.currentTimeMillis())))
}

[info] running fs2.PullSleepApp 
1625160228568
1625160229692