message: add convertion to u64 and from u32
diff: @@ -142,6 +142,13 @@ impl ops::Sub<Coin> for Result<Coin> {
  }
  }
  +impl From<Coin> for u64 {
  + fn from(c: Coin) -> u64 { c.0 }
  +}
  +
  +impl From<u32> for Coin {
  + fn from(c: u32) -> Coin { Coin(c as u64) }
  +}
  pub fn sum_coins(coins: &[Coin]) -> Result<Coin> {
  coins.iter().fold(Coin::new(0), |acc, ref c| acc.and_then(|v| v + *c))
  }
  