message: Add block size to write_block signature.
  This allows the elision of bounds checking.
diff: @@ -215,7 +215,7 @@ impl<'a, W: Write + 'a> BitWriter<'a, W> {
  fn write_block(
  &mut self,
  - block: &[i32],
  + block: &[i32; 64],
  prevdc: i32,
  dctable: &[(u8, u16)],
  actable: &[(u8, u16)],
  