Surfacing submarines

[durban]

(This might already be obvious to all involved, in which case let's make this a documentation issue.)

It is trivially easy to surface a submarine exception:

  def foo[F[_]](implicit F: Concurrent[F]): F[Unit] = {
    Handle.allowF[F, String] { handle =>
      F.start(handle.raise("foo"))
    }.rescue(_ => F.never).flatMap[Unit] { fib =>
      F.widen(fib.joinWithNever)
    }
  }

The idea is that we can use the Handle in a completely different fiber, while allowF/rescue is lexical.