Make `PublicKeySet` orderable, too.

src/lib.rs

@@ -526,7 +526,7 @@ impl Hash for DecryptionShare {
 }
 
 /// A public key and an associated set of public key shares.
-#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq)]
+#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq, Ord, PartialOrd)]
 pub struct PublicKeySet {
     /// The coefficients of a polynomial whose value at `0` is the "master key", and value at
     /// `i + 1` is key share number `i`.