Changers

[Yuriy Skalko]

> I still prefer noChange() or nullChange() to changeNone(), but we would have to ask a native English speaker.

I proposed `changeNone` to have consistent naming for all 
change`Something` functions. But noChange is also good. If there will be 
any comments from native speakers, it can be renamed any moment.

> Concerning changeRef, I would say that the fact that this is a reference is an implementation detail. Maybe change(ref, val) is good enough, or else changeVar(ref, val).

Agree, changeVar is better than changeRef. `change` is too common word 
to use it alone, so better have `Var` suffix.

> Sorry for the nitpicking, but since this Changer stuff is not intuitive at first sight, it is nice to have clear names.
> 
> JMarc

Not a problem. Good naming can make code really intuitive and easy to 
understand. It is worth to spend some time on this.

I've committed the updated patch.

Yuriy