Use C++20 in the C++ wrapper

This commit mostly just fixes compilation errors when compiling in C++20 mode, though the change to std::filesystem::path::u8string()'s return type means it's not backwards-compatible with C++17.

cpp/build.rs

@@ -1,6 +1,6 @@
 fn main() {
     cxx_build::bridge("src/lib.rs")
-        .std("c++17")
+        .std("c++20")
         .flag_if_supported("/Zc:__cplusplus")
         .flag_if_supported("/permissive-")
         .compile("libloot-cpp");