Add some top-level examples (#303)

The examples show:
- Building a Signed Pod with different types of values
- Building a MainPod
- Input SignedPod to MainPod
- Input MainPod to MainPod
- Using MainPod or MockMainPod
- Using custom predicates

src/examples/mod.rs

@@ -155,7 +155,7 @@ impl EthDosHelper {
 
         let mut pod = MainPodBuilder::new(&self.params, &self.vd_set);
         pod.add_signed_pod(int_attestation);
-        pod.add_main_pod(eth_dos_src_to_int_pod.clone());
+        pod.add_recursive_pod(eth_dos_src_to_int_pod.clone());
 
         let eth_dos_int_to_dst = eth_dos_src_to_int_pod
             .pod