message: Clean up build_scan_header
diff: @@ -742,19 +742,15 @@ fn build_frame_header(
  fn build_scan_header(m: &mut Vec<u8>, components: &[Component]) {
  m.clear();
  - // TODO: More idiomatic would be extend_from_slice, to_be_bytes
  - let _ = m.write_all(&[components.len() as u8]);
  + let _ = m.push(components.len() as u8);
  for &comp in components.iter() {
  - let _ = m.write_all(&[comp.id]);
  let tables = (comp.dc_table << 4) | comp.ac_table;
  - let _ = m.write_all(&[tables]);
  + let _ = m.extend_from_slice(&[comp.id, tables]);
  }
  // spectral start and end, approx. high and low
  - let _ = m.write_all(&[0]);
  - let _ = m.write_all(&[63]);
  - let _ = m.write_all(&[0]);
  + m.extend_from_slice(&[0, 63, 0]);
  }
  fn build_huffman_segment(
  