Implement the mul. of two Fausts in C++ and bind py. wrapper to this impl.

- Fix by the way FaustCoreCpp::to_string(), avoiding to use directly c_str() on the result of iostringstream::str() like described in the notes here: https://en.cppreference.com/w/cpp/io/basic_ostringstream/str. It actually made strange results, empty strings and utf-8 decoding error.