String stream objects

Reference-counted versions of string stream objects: x::istringstream, x::ostringstream, x::stringstream, and their wide character counterparts, x::wistringstream, x::wostringstream, x::wstringstream, examples:

#include <x/basicstringstreamobj.H>

x::ostringstream o(x::ostringstream::create());

(*o) << "Grand total: " << n;

// ...

std::string str;

x::istringstream i(x::istringstream::create(str));

i >> n;