Integrate recursion into MainPod (#243)

* calculate MainPod id in a dynamic-friendly way

The MainPod id is now calculated with front padding and a fixed size
independent of max_public_statements so that introduction gadgets can be
verified by a MainPod while paying only for the number of statements
they use.  This is because with front padding of none-statements we can
precompute the poseidon state corresponding to absorbing all the padding
statements and only pay constraints for the non-padding statements.

The id is calculated as follows:
`id = hash(serialize(reverse(statements || none-statements)))`

* add time feature and disable timing by default

* apply suggestions from @arnaucube

* link issues in todos

src/backends/plonky2/mainpod/statement.rs

@@ -1,4 +1,4 @@
-use std::fmt;
+use std::{fmt, iter};
 
 use serde::{Deserialize, Serialize};
 
@@ -28,9 +28,15 @@ impl Statement {
 }
 
 impl ToFields for Statement {
-    fn to_fields(&self, _params: &Params) -> Vec<middleware::F> {
-        let mut fields = self.0.to_fields(_params);
-        fields.extend(self.1.iter().flat_map(|arg| arg.to_fields(_params)));
+    fn to_fields(&self, params: &Params) -> Vec<middleware::F> {
+        let mut fields = self.0.to_fields(params);
+        fields.extend(
+            self.1
+                .iter()
+                .chain(iter::repeat(&StatementArg::None))
+                .take(params.max_statement_args)
+                .flat_map(|arg| arg.to_fields(params)),
+        );
         fields
     }
 }