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.

src/xenia/premake5.lua

@@ -7,6 +7,7 @@ project("xenia-core")
   kind("StaticLib")
   language("C++")
   links({
+    "fmt",
     "xenia-base",
   })
   defines({