message: expose DerivationIndex
diff: @@ -614,7 +614,7 @@ impl<'de, T> serde::Deserialize<'de> for Signature<T>
  pub type ChainCode = [u8; CHAIN_CODE_SIZE];
  -type DerivationIndex = u32;
  +pub type DerivationIndex = u32;
  #[derive(Debug, PartialEq, Eq, Deserialize, Serialize)]
  enum DerivationType {
  