Put variable-length fields last (#103)

This helps prevent parsing issues in other implementations.

src/frost.rs

@@ -100,15 +100,15 @@ pub struct GroupCommitment(jubjub::ExtendedPoint);
 /// To derive a FROST keypair, the receiver of the [`SharePackage`] *must* call
 /// .into(), which under the hood also performs validation.
 pub struct SharePackage {
+    /// The public signing key that represents the entire group.
+    pub(crate) group_public: VerificationKey<SpendAuth>,
     /// Denotes the participant index each share is owned by. We implicitly
     /// restrict the number of participants to 255.
     pub index: u8,
-    /// This participant's share.
-    pub(crate) share: Share,
     /// This participant's public key.
     pub(crate) public: Public,
-    /// The public signing key that represents the entire group.
-    pub(crate) group_public: VerificationKey<SpendAuth>,
+    /// This participant's share.
+    pub(crate) share: Share,
 }
 
 impl TryFrom<SharePackage> for KeyPackage {
@@ -379,11 +379,13 @@ impl From<(u8, &SigningNonces)> for SigningCommitments {
 /// Generated by the coordinator of the signing operation and distributed to
 /// each signing party.
 pub struct SigningPackage {
-    /// Message which each participant will sign
-    pub message: &'static [u8],
     /// The set of commitments participants published in the first round of the
     /// protocol.
     pub signing_commitments: Vec<SigningCommitments>,
+    /// Message which each participant will sign.
+    ///
+    /// Each signer should perform protocol-specific verification on the message.
+    pub message: &'static [u8],
 }
 
 /// A participant's signature share, which the coordinator will use to aggregate