streams: cache file position within AutoFile

src/streams.h

@@ -390,9 +390,10 @@ class AutoFile
 protected:
     std::FILE* m_file;
     std::vector<std::byte> m_xor;
+    std::optional<int64_t> m_position;
 
 public:
-    explicit AutoFile(std::FILE* file, std::vector<std::byte> data_xor={}) : m_file{file}, m_xor{std::move(data_xor)} {}
+    explicit AutoFile(std::FILE* file, std::vector<std::byte> data_xor={});
 
     ~AutoFile() { fclose(); }