Added more utility and fixed Flag type.

source/mijin/io/stream.hpp

@@ -134,6 +134,9 @@ public:
     ~FileStream() override;
 
     StreamError open(const char* path, FileOpenMode mode_);
+    inline StreamError open(const std::string& path, FileOpenMode mode_) {
+        return open(path.c_str(), mode_);
+    }
     void close();
     [[nodiscard]] inline bool isOpen() const { return handle != nullptr; }