[v3] add_unsigned/remove_unsigned tweaks

[Paolo Carlini]

Howard Hinnant wrote:

> I'm asking because __add_unsigned builds on __is_integer and impacts  
> how we handle __add_unsigned with cv-qualified integral types.

In my opinion, the "new traits" should behave consistently with the 
other traits in TR1 and the working paper ones wrt cv-qualification. On 
the other hand, at present, it's not an option using 
std::tr1::is_integral and other non-uglified names in the implementation 
of ext/type_traits.h (supposedly available also to the users of the 
library) and the legacy cpp_type_traits.h.

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.

Paolo.