[v3] add_unsigned/remove_unsigned tweaks

[Paolo Carlini]

Paolo Carlini wrote:

> All in all, probably the best way to go is using something like your 
> clever __union_cv for __add_unsigned and possibly, as a backward 
> compatible change, for cpp_type_traits traits, it would not add so 
> much code, after all.

Looking better, a practical problem of course is that the implementation 
of __union_cv needs various traits, such as is_const, is_volatile, which 
are not available in uglified form, in cpp_type_traits... That is a 
problem, a very serious one.

Paolo.