General improvements (#56)

* Apply Rust traits to all public types and other improvements

* Move methods into appropriate section

* Check for zero scalars

* Change element and scalar de/serialization from `GenericArray` to slice

* Customize `serde` serialization

src/error.rs

@@ -27,6 +27,8 @@ pub enum Error {
     ProofVerification,
     /// Size of seed is longer then [`u16::MAX`].
     Seed,
+    /// The protocol has failed and can't be completed.
+    Protocol,
 }
 
 /// Only used to implement [`Group`](crate::Group).