std::string overloads for xfstream

[Jonathan Wakely]

2009/3/17 Ed Smith-Rowland:
>
> We generally know about the big bits of C++-0X and are making rapid
> progress.  But there are probably lots of crumbs like this one that are not
> captured in the status pages.  (e.g. I know that valarray needs the rvalue
> swap method and namespace swap functions and rvalue ctors and assigns).

The rvalue swap functions are being removed, so don't spend any time
adding them.

Jon