FIX: support boost 1.81

[mtasaka]

https://www.boost.org/users/history/version_1_79_0.html

boost 1.79 deprecated boost/filesystem/string_file.hpp and with boost 1.81 boost::filesystem::ofstream or so is no longer available.

Replace these with boost::nowide in boost/nowide/fstream.hpp .

Fixes #610 .

[manuel5975p]

Is this getting merged? It's uncompilable as is