sso tradeoffs

[Howard Hinnant]

This is just a heads up, and probably an obvious one at that, but I  
thought I'd err on the side of repetitiveness:

When we do an sso string (as the official std::string), we at the  
same time need an extension non-mutable string whose copy constructor  
can't throw (probably via refcounting).  This will be needed in the  
<stdexcept> classes (which currently use std::string).

Hmm... unless we can get the exception throwing mechanism in the  
language to promise to always move instead of copy...

-Howard