Implement `tracked` members

[KacperFKorban]

It would be great to be able to declare abstract tracked members.

tracked members, when overridden, will keep their exact type.

e.g. the following code should compile

import scala.language.experimental.modularity
import scala.language.future

trait F:
  tracked val x: Int

object Test:
  val f = new F:
    val x = 1

  summon[f.x.type <:< 1]