Setting the state change waker must take effect for all existing state Vars too

[zetanumbers]

This needs some change in public API that would also make sense. I think we should require to specify some kind of reference to the waker for each passed function. Should help resolve issue #215.

[anp]

If I understand correctly from our Discord conversation, the issue you're seeing looks like:

1. set a runtime R1's waker to W1
2. call R1.run_once(), creating some cached values and Vars
3. calls to Var::update wake W1
4. call R1.set_state_change_waker(W2)
5. calls to Var::update should wake W2 but don't

Is that correct? If so this would also be an issue even if you called R1.run_once() after step 4 but before step 5.