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/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)