Add AutoFile::seek and tell

It's useful to be able to seek to a specific position in a file. Allow
AutoFile to seek by using fseek.

It's also useful to be able to get the current position in a file.
Allow AutoFile to tell by using ftell.

src/streams.h

@@ -435,6 +435,9 @@ public:
     /** Implementation detail, only used internally. */
     std::size_t detail_fread(Span<std::byte> dst);
 
+    void seek(int64_t offset, int origin);
+    int64_t tell();
+
     //
     // Stream subset
     //