pub fn seek_relative(self: Pin<&mut Self>, offset: i64) -> SeekRelative<'_, R> {
        SeekRelative { inner: self, offset, first: true }
    }
Seeks relative to the current position. If the new position lies within the buffer,
the buffer will not be flushed, allowing for more efficient seeks.
This method does not return the location of the underlying reader, so the caller
must track this information themselves if it is required.
futures-util/src/io/buf_reader.rs