untrackedCaptures

Three 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 root capability in its capture set, so that the root capability is not contributed to the class instance. Example:

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

has type A^ since b contributes an any. But

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

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

3. Allows a field to be declarewd in a class that does not extend Stateful, and suppresses checks for updates to the field.