message: fix compilation of the tests
diff: /// ```
  /// use wallet_crypto::bip44::{Account, Change, Addressing};
  ///
  -/// let addr = Account::new(0).unwrap()
  +/// let scheme_value = Account::new(0).unwrap()
  /// .external().unwrap()
  -/// .get_scheme_value(0).unwrap();
  +/// .get_scheme_value();
  ///
  -/// assert!(addr.index == 0);
  +/// assert!(scheme_value == 0);
  /// ```
  use hdpayload::{Path};
  @@ -239,6 +239,7 @@ impl Change {
  if change >= 0x80000000 { return Err(Error::ChangeOutOfBound(change)); }
  Ok(Change{ account: account, change: change })
  }
  + pub fn get_scheme_value(&self) -> u32 { self.change }
  pub fn index(&self, index: u32) -> Result<Addressing> {
  Addressing::new_from_change(*self, index)
  @@ -324,13 +325,13 @@ impl Addressing {
  /// # Example
  ///
  /// ```
  - /// use wallet_crypto::bip44::{Addressing, AddrType};
  + /// use wallet_crypto::bip44::{Addressing, AddrType, Index};
  ///
  /// let addr = Addressing::new(0, AddrType::External).unwrap();
  ///
  /// let next = addr.incr(32).unwrap().incr(10).unwrap();
  ///
  - /// assert!(next.index == 42);
  + /// assert!(next.index == Index::new(42).unwrap());
  /// assert!(next.incr(0x80000000).is_err());
  /// ```
  pub fn incr(&self, incr: u32) -> Result<Self> {
  