Handling unicode paths

As far as I can see, it is not possible to use paths on Windows such as `C:\Överkörd\Überförare\`, maybe paths should be considered UTF-8, then convert to wchar_t* for use with std::ofstream/ifstream.

Start by reproducing the reported Windows failure with the Unicode path C:\Överkörd\Überförare\ and inspect the code that passes paths to std::ofstream and std::ifstream. Done means the library can open and use such paths on Windows without breaking existing path handling.

Written by the indexing model from the issue text.