message: Define MAX_SIZE constant for CompactSize serialization
diff: use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
  use std::io::{self, Read, Write};
  +const MAX_SIZE: usize = 0x02000000;
  +
  struct CompactSize;
  impl CompactSize {
  @@ -33,7 +35,7 @@ impl CompactSize {
  n => Ok(n as usize),
  }
  }? {
  - s if s > 0x02000000 => Err(io::Error::new(
  + s if s > MAX_SIZE => Err(io::Error::new(
  io::ErrorKind::InvalidInput,
  "CompactSize too large",
  )),
  