std::string overloads for xfstream

[Paolo Carlini]

Ed Smith-Rowland wrote:
> All,
>
> This is a new patch and ChangeLog.
Much better, thanks. Can you please remove once again the trailing blank
lines, avoid the inclusion of <testsuite_hooks.h> and the test variable
dance (redundant for compile-only testcases), and also, I think, avoid
mentioning the section of the standard, most likely incorrect in C++0x?

Thanks,
Paolo.