Make fs.h C++20 compliant

Without the changes, the file will fail to compile under C++20 because
char8_t can not be converted to char implicitly.

src/fs.h

@@ -51,12 +51,26 @@ public:
     // Disallow std::string conversion method to avoid locale-dependent encoding on windows.
     std::string string() const = delete;
 
+    std::string u8string() const
+    {
+        const auto& utf8_str{std::filesystem::path::u8string()};
+        // utf8_str might either be std::string (C++17) or std::u8string
+        // (C++20). Convert both to std::string. This method can be removed
+        // after switching to C++20.
+        return std::string{utf8_str.begin(), utf8_str.end()};
+    }
+
     // Required for path overloads in <fstream>.
     // See https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=96e0367ead5d8dcac3bec2865582e76e2fbab190
     path& make_preferred() { std::filesystem::path::make_preferred(); return *this; }
     path filename() const { return std::filesystem::path::filename(); }
 };
 
+static inline path u8path(const std::string& utf8_str)
+{
+    return std::filesystem::u8path(utf8_str);
+}
+
 // Disallow implicit std::string conversion for absolute to avoid
 // locale-dependent encoding on windows.
 static inline path absolute(const path& p)
@@ -116,8 +130,8 @@ static inline std::string PathToString(const path& path)
     // use here, because these methods encode the path using C++'s narrow
     // multibyte encoding, which on Windows corresponds to the current "code
     // page", which is unpredictable and typically not able to represent all
-    // valid paths. So std::filesystem::path::u8string() and
-    // std::filesystem::u8path() functions are used instead on Windows. On
+    // valid paths. So fs::path::u8string() and
+    // fs::u8path() functions are used instead on Windows. On
     // POSIX, u8string/u8path functions are not safe to use because paths are
     // not always valid UTF-8, so plain string methods which do not transform
     // the path there are used.