Merge the two `impl` blocks for PNGImage

rust/coffi/src/main.rs

        return img
    }
    fn begin_read_from_file(&mut self, file_name: *const libc::c_char) -> u32 {
        unsafe { png_image_begin_read_from_file(self, file_name) as u32 }
    }
}
impl PNGImage {
    fn message(&self) -> String {
        String::from_utf8(self.message.iter().map(|&c| c as u8).collect()).unwrap()
    }
    fn begin_read_from_file(&mut self, file_name: *const libc::c_char) -> u32 {
        unsafe { png_image_begin_read_from_file(self, file_name) as u32 }
    }
}
impl std::fmt::Display for PNGImage {