std::string overloads for xfstream

[Paolo Carlini]

Hi,
> Here is another round.  I removed the mains in the testcases.
>
> I'm positive all the extra terminal whitespace has been removed.
> I did 'egrep " +$" ' on everything and checked in my editor too there was no terminating whitespece in the last patch.
>   
For the record, the trailing blank lines are still there (I didn't say
whitespace) but don't worry I'll remove them, patch seems otherwise ok.

Paolo.