limit the number of StatementTmpl in CustomPredicate: (#101)

* limit the number of StatementTmpl in CustomPredicate:

- add constructor method for CustomPredicate
- make size checks at the CustomPredicate creation, so that once instantiated we can assume that contains valid data

This resolves #79

* Update tests to use new interface

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Co-authored-by: Ahmad <root@ahmadafuni.com>

src/frontend/operation.rs

@@ -1,7 +1,7 @@
 use std::fmt;
 
 use super::{AnchoredKey, SignedPod, Statement, StatementArg, Value};
-use crate::middleware::{hash_str, NativeOperation, NativePredicate, OperationType, Predicate};
+use crate::middleware::{hash_str, NativePredicate, OperationType, Predicate};
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum OperationArg {