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/backends/plonky2/mock_main/mod.rs

@@ -435,7 +435,7 @@ impl Pod for MockMainPod {
                 self.operations[i]
                     .deref(&self.statements[..input_statement_offset + i])
                     .unwrap()
-                    .check(&s.clone().try_into().unwrap())
+                    .check(&self.params, &s.clone().try_into().unwrap())
             })
             .collect::<Result<Vec<_>>>()
             .unwrap();