Make `io::BorrowedCursor::advance` safe This also keeps the old `advance` method under `advance_unchecked` name. This makes pattern like `std::io::default_read_buf` safe to write.
Make `io::BorrowedCursor::advance` safe This also keeps the old `advance` method under `advance_unchecked` name. This makes pattern like `std::io::default_read_buf` safe to write.