message: 1. Use a base64::encode instead of hex::encode for converting params
  between messages.
  2. Added a little test for conversions between UnsignedMessage and
  UnsignedMessageAPI
diff: @@ -33,6 +33,7 @@ use secp256k1::util::{
  COMPRESSED_PUBLIC_KEY_SIZE, FULL_PUBLIC_KEY_SIZE, SECRET_KEY_SIZE, SIGNATURE_SIZE,
  };
  use secp256k1::{recover, sign, verify, Message, RecoveryId};
  +use zeroize::Zeroize;
  use zx_bip44::BIP44Path;
  use crate::signature::{Signature, SignatureBLS, SignatureSECP256K1};
  @@ -58,6 +59,8 @@ impl AsRef<[u8]> for CborBuffer {
  pub const SIGNATURE_RECOVERY_SIZE: usize = SIGNATURE_SIZE + 1;
  /// Private key buffer
  +#[derive(Zeroize, Debug)]
  +#[zeroize(drop)]
  pub struct PrivateKey(pub [u8; SECRET_KEY_SIZE]);
  /// Public key buffer
  