std::wstring support

It seems std::wstring is not supported by the x2struct library. Is there any plan to implement that?

struct MYSTRUCT
{
    std::wstring content;
    XTOSTRUCT(O(content))
};

Failed to build doe to a x_struct_to_str function not found.
Replacing the wstring to string can pass the build, but failed to process wide characters.

https://stackoverflow.com/a/7154226/8364323

[xyz347]

will support later