message: add a as_ref for Mnemonics
diff: @@ -630,6 +630,7 @@ pub const MAX_MNEMONIC_VALUE : u16 = 2047;
  ///
  #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Copy, Clone)]
  pub struct MnemonicIndex(pub u16);
  +
  impl MnemonicIndex {
  /// smart constructor, validate the given value fits the mnemonic index
  /// boundaries (see [`MAX_MNEMONIC_VALUE`](./constant.MAX_MNEMONIC_VALUE.html)).
  @@ -704,6 +705,13 @@ impl MnemonicIndex {
  ///
  #[derive(Debug, PartialEq, Eq, Clone)]
  pub struct Mnemonics(Vec<MnemonicIndex>);
  +
  +impl AsRef<[MnemonicIndex]> for Mnemonics {
  + fn as_ref(&self) -> &[MnemonicIndex] {
  + &self.0[..]
  + }
  +}
  +
  impl Mnemonics {
  /// get the [`Type`](./enum.Type.html) of this given `Mnemonics`.
  ///
  