Queued but unstarted work on `Dispatcher.sequential` should be cancelable

A queued task to a dispatcher cannot be canceled:

import cats.effect.std.Dispatcher
import cats.effect.unsafe.implicits.global
import cats.effect.IO
import scala.concurrent.Await
import scala.concurrent.duration.Duration

val (dispatcher, shutDownDispatcher) = Dispatcher.sequential[IO].allocated.unsafeRunSync()

dispatcher.unsafeToFuture(IO.never)
val cancel = dispatcher.unsafeRunCancelable(IO.unit)
Await.result(cancel(), Duration.Inf) // hangs

Perhaps this is explained somewhere, but it was surprising to me.

Start by running the Scala reproduction in the issue and inspect the Dispatcher.sequential, unsafeToFuture, and unsafeRunCancelable entry points. Trace how queued work is handled and compare the behavior of the returned cancel function. Done means the queued task can be canceled without the call hanging, with coverage for this reproduction.