[Patch] Export more basic_string symbols

[Paolo Carlini]

Benjamin Kosnik wrote:

>Let's just remove the std::string, std::basic_string export
>restrictions. I'd put them in so that operator+ could be added in 3.2,
>3.3.
>
Great!

Thanks,
Paolo.

P.S. In the next days (while you are completing the tough 27_io work) 
I'd like to provide some basic_string tests which are currently missing 
such as those for 'find_last_of' and 'find_last_not_of'.