C++17ification.

C++17ification!

- Filesystem interaction now uses std::filesystem::path.
- Usage of const char*, std::string have been changed to
  std::string_view where appropriate.
- Usage of printf-style functions changed to use fmt.

third_party/mspack/logging.cc

@@ -1,5 +1,7 @@
 #include "xenia/base/logging.h"
 
+#include <cstdarg>
+
 extern "C" void xenia_log(const char* fmt, ...) {
   char buffer[128];
   va_list args;