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/custom.rs

@@ -4,7 +4,6 @@ use std::{collections::HashMap, fmt, hash as h, iter, iter::zip, sync::Arc};
 use anyhow::{anyhow, Result};
 use schemars::JsonSchema;
 
-// use serde::{Deserialize, Serialize};
 use crate::{
     frontend::{AnchoredKey, Statement, StatementArg},
     middleware::{