MutableOf etc and QualifierOf

[Shriramana Sharma via Digitalmars-d]

The templates `MutableOf` etc in std.traits – they don't seem to be of any 
public use since `const(T)` is more direct and meaningful than `ConstOf!T`. 

Except for `MutableOf` they are all currently public and even the `package`-
level `MutableOf` is publicly documented. All this seems pointless.

Of course they are used in the implementation of std.variant but for that 
they all can be marked `package` and they don't need to be publicly 
documented.

They are also used for making the internal life of QualifierOf easier, but 
QualifierOf itself is not publicly documented.

Is QualifierOf intended to be publicly used? What is the difference in final 
utility between QualifierOf and CopyTypeQualifiers?

Thanks!

-- 
Shriramana Sharma, Penguin #395953