Implement `tracked` members

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

```scala
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]
``` 