proxifying Subjects

[kosich]

Subjects should be proxifiable as well, letting us do:

const o = new Subject<{ value: number }>();
const p = proxify(o);

p.subscribe(console.log);
p.value.subscribe(console.log);
p.next({ value: 0 });
p.value.next(1);

Same should apply to BehaviorSubject and ReplaySubject

[kosich]

fixed with f9fc24d