add eq & partial_eq for RecursivePod, Pod traits and VDSet struct (for usage from introduction-pods) (#309)

* add eq & partial_eq for VDSet (for usage from introduction-pods)

* add eq & partial_eq impls for Pod & RecursivePod traits

src/backends/plonky2/mainpod/mod.rs

@@ -561,7 +561,7 @@ impl PodProver for Prover {
     }
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, PartialEq, Eq)]
 pub struct MainPod {
     params: Params,
     id: PodId,
@@ -676,6 +676,17 @@ impl Pod for MainPod {
         })
         .expect("serialization to json")
     }
+
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+    fn equals(&self, other: &dyn Pod) -> bool {
+        if let Some(other) = other.as_any().downcast_ref::<MainPod>() {
+            self == other
+        } else {
+            false
+        }
+    }
 }
 
 impl RecursivePod for MainPod {

src/backends/plonky2/mainpod/statement.rs

@@ -10,6 +10,8 @@ use crate::{
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct Statement(pub Predicate, pub Vec<StatementArg>);
 
+impl Eq for Statement {}
+
 impl Statement {
     pub fn is_none(&self) -> bool {
         self.0 == Predicate::Native(NativePredicate::None)