Fix custom predicate circuits and add tests for them (#235)

* add tests, fix custom predicates

* wip

* wip

* fix custom predicates

* modularize code

* fix typos

* remove scratch file

* update

* Update src/backends/plonky2/circuits/mainpod.rs

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

---------

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

src/middleware/custom.rs

@@ -276,9 +276,9 @@ impl CustomPredicate {
     pub fn pad_statement_tmpl(&self) -> StatementTmpl {
         StatementTmpl {
             pred: Predicate::Native(if self.conjunction {
-                NativePredicate::False
-            } else {
                 NativePredicate::None
+            } else {
+                NativePredicate::False
             }),
             args: vec![],
         }