message: Optimize write_block.
  Removing some spurious code and reading values directly from UNZIGZAG generates a 3-8% performance boost
diff: @@ -231,8 +231,8 @@ impl<'a, W: Write + 'a> BitWriter<'a, W> {
  // Figure F.2
  let mut zero_run = 0;
  - for k in 1usize..=63 {
  - if block[UNZIGZAG[k] as usize] == 0 {
  + for &k in &UNZIGZAG[1..] {
  + if block[k as usize] == 0 {
  zero_run += 1;
  } else {
  while zero_run > 15 {
  @@ -240,17 +240,13 @@ impl<'a, W: Write + 'a> BitWriter<'a, W> {
  zero_run -= 16;
  }
  - let (size, value) = encode_coefficient(block[UNZIGZAG[k] as usize]);
  + let (size, value) = encode_coefficient(block[k as usize]);
  let symbol = (zero_run << 4) | size;
  self.huffman_encode(symbol, actable)?;
  self.write_bits(value, size)?;
  zero_run = 0;
  -
  - if k == 63 {
  - break;
  - }
  }
  }
  