untrackedCaptures

Two usages:

1. Marks the constructor parameter as untracked. The capture set of this parameter will not be included in the capture set of the constructed object.

2. Marks a class field that has a cap in its capture set, so that the cap is not contributed to the class instance. Example:

   class A { val b B^ = ... }; new A()
   

   has type A^ since b contributes a cap. But

   class A { @untrackedCaptures val b: B^ = ... }; new A()
   

   has type A. The b field does not contribute its cap.