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/mod.rs

@@ -1,5 +1,6 @@
 pub mod basetypes;
 pub mod circuits;
+pub mod emptypod;
 mod error;
 pub mod mainpod;
 pub mod mock;
@@ -7,4 +8,32 @@ pub mod primitives;
 pub mod recursion;
 pub mod signedpod;
 
+use std::sync::LazyLock;
+
 pub use error::*;
+
+use crate::{
+    backends::plonky2::{
+        basetypes::CircuitData,
+        circuits::mainpod::{MainPodVerifyTarget, NUM_PUBLIC_INPUTS},
+        recursion::RecursiveCircuit,
+    },
+    middleware::Params,
+    timed,
+};
+
+pub static DEFAULT_PARAMS: LazyLock<Params> = LazyLock::new(Params::default);
+
+pub static STANDARD_REC_MAIN_POD_CIRCUIT_DATA: LazyLock<CircuitData> = LazyLock::new(|| {
+    let params = &*DEFAULT_PARAMS;
+    timed!(
+        "recursive MainPod circuit_data",
+        RecursiveCircuit::<MainPodVerifyTarget>::target_and_circuit_data(
+            params.max_input_recursive_pods,
+            NUM_PUBLIC_INPUTS,
+            params
+        )
+        .expect("calculate circuit_data")
+        .1
+    )
+});