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

---------

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

src/middleware/mod.rs

@@ -92,6 +92,22 @@ pub struct Params {
     pub max_custom_batch_size: usize,
 }
 
+impl Default for Params {
+    fn default() -> Self {
+        Self {
+            max_input_signed_pods: 3,
+            max_input_main_pods: 3,
+            max_statements: 20,
+            max_signed_pod_values: 8,
+            max_public_statements: 10,
+            max_statement_args: 5,
+            max_operation_args: 5,
+            max_custom_predicate_arity: 5,
+            max_custom_batch_size: 5,
+        }
+    }
+}
+
 impl Params {
     pub fn max_priv_statements(&self) -> usize {
         self.max_statements - self.max_public_statements
@@ -134,22 +150,6 @@ impl Params {
     }
 }
 
-impl Default for Params {
-    fn default() -> Self {
-        Self {
-            max_input_signed_pods: 3,
-            max_input_main_pods: 3,
-            max_statements: 20,
-            max_signed_pod_values: 8,
-            max_public_statements: 10,
-            max_statement_args: 5,
-            max_operation_args: 5,
-            max_custom_predicate_arity: 5,
-            max_custom_batch_size: 5,
-        }
-    }
-}
-
 pub trait Pod: fmt::Debug + DynClone {
     fn verify(&self) -> bool;
     fn id(&self) -> PodId;