Re-implement serialization (#201)

* Serialization tests now pass again

* Tidy up and test more edge-cases

* Use attributes rather than custom serializer for arrays

* Add JSON Schema support

* Tests for JSON Schema generation and validation

* Add comments

* Support custom predicates

* Clippy fixes

* Make deserialization/constructor functions pub(crate)

src/frontend/operation.rs

@@ -1,6 +1,5 @@
 use std::fmt;
 
-// use serde::{Deserialize, Serialize};
 use crate::{
     frontend::SignedPod,
     middleware::{AnchoredKey, OperationAux, OperationType, Statement, Value},