message: Better debug for Datagram
  The existing debug statement drops an array, which is ugly and hard to
  use.  This uses hex().
diff: use std::net::SocketAddr;
  use std::ops::Deref;
  -#[derive(Debug, PartialEq, Clone)]
  +use crate::hex;
  +
  +#[derive(PartialEq, Clone)]
  pub struct Datagram {
  src: SocketAddr,
  dst: SocketAddr,
  @@ -41,3 +43,15 @@ impl Deref for Datagram {
  &self.d
  }
  }
  +
  +impl std::fmt::Debug for Datagram {
  + fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
  + write!(
  + f,
  + "Datagram {:?}->{:?}: {}",
  + self.src,
  + self.dst,
  + hex(&self.d)
  + )
  + }
  +}
  