Adding a new design constraint to D

[The Zealot]

> The main difference between @mustuse and private(scope) is that 
> @mustuse was *impossible* to simulate using existing language 
> features, whereas private(scope) can be simulated by extracting 
> the relevant code into its own dedicated module. I understand 
> that you have reasons for disliking this workaround, but the 
> fact that a workaround exists is still relevant when 
> determining what ought to be prioritized.

I don't think calling it a workaround is even correct.
_class private_ would actually be the workaround. Just to put 
multiple classes inside one file that should conceptually not be 
in the same file in the first place.