Specializations of std::char_traits

[B. Kosnik]

>  std::copy(__s2, __s2 + __n, __s1);

> std::fill_n(__s, __n, __a);

Nice. You should make this change to pod_char_traits.h too.

> On the contrary, I think it does matter.  int_type is supposed to be
> provided by char_traits<>.  Nobody knows whether it is used by users
> only with iostreams.  Either we provide something consistent or we don't
> provide a definition at all.

Ditto for state_type. I don't see how a truly generic char_traits can be
done with one template parameter: I'd think (at least) three would be
necessary.

-benjamin