message: wallet::rindex: Add method RootKey::into_xprv()
  This comes handy in code that needs to extract the inner XPrv value
  from a RootKey that can be moved, rather than doing `(*root_key).clone()`.
diff: @@ -315,6 +315,11 @@ impl RootKey {
  Ok(RootKey::new(xprv, derivation_scheme))
  }
  + /// Converts into the inner `XPrv` value
  + pub fn into_xprv(self) -> XPrv {
  + self.root_key
  + }
  +
  pub fn address_generator(&self) -> AddressGenerator<XPrv>
  {
  AddressGenerator::<XPrv>::new(self.root_key.clone(), self.derivation_scheme)
  