Type traits

[Gabriel Dos Reis]

Hi,

I added bits/cpp_type_traits.h to current CVS.  Its main purpose is to
provide some type information in form of integral constant usable in
control structures (as to oppose to stl/bits/type_traits.h).  Those
integral constant expressions are ideally used as *explicit* template
arguemnts. 

After this, I think the do-the-right-thing std::string template
constructor should be straightfoward to implement.

Comments appreciated.

-- Gaby