Feat/disk cache (#354)

- Bump rust version to `nightly-2025-07-02` because some of the nightly features we were using have been stabilized.
- Introduce feature `disk_cache` which enables caching to disk.  Each time an artifact is retrieved from the cache it will be read and deserialized.  On a cache miss the artifact will be created, serialized and stored to disk.
- Introduce feature `mem_cache` which enables caching to memory.  All cached artifacts are kept in memory after they are created.  The mem cache implementation avoids cloning of artifacts by extending their lifetime to `'static`.  This is `unsafe` code, but I argue that this usage is safe.
- Add a `build.rs`
  - When the feature `disk_cache` is enabled, the `build.rs` will inject env variables to the process with the git commit information, which is used to index the cached artifacts
- Replace all previous cached artifacts from `LazyStatic` methods that call the cache API
- Derive `Serialize, Deserialize` for all `*Target` types so that they can be serialized for caching to disk
- Add finer level of caching: now we cache the `CircuitData` and `VerifierData` independently.  The reason for this is that `CircuitData` is a very big artifact which is not needed for verification.  So by only accessing `VerifierData` in verification we don't pay a big overhead for reading from disk and deserializing
- Add missing artifacts to the cache: like the `CircuitData` for the `MainPod` indexed by `Params`
- Add helper types to serialize and deserialize `CircuitData`, `CommonData` and `VerifierData` with the set of gates and generators used in the recursive MainPod circuit
- Tweak the ids of our custom gates so that they remain unique when their generic parameters change
- Bugfix: several tests were using the standard `vd_set` but were using MainPod circuits with non-default parameters.  This was working before because there was a bug: the MainPod circuit was reporting that the used verifier data was the standard one instead of picking the one corresponding to it's own Params.

Summary of breaking changes:
- One and only one of the features `mem_cache` or `disk_cache` need to be enabled.  By default it's `mem_cache`
  - To enable the `disk_cache` you need to disable the default features like this: `--no-default-features --features=backend_plonky2,zk,disk_cache`
- Removed `DEFAULT_PARAMS`, instead use `Params::default()`
- Removed `STANDARD_REC_MAIN_POD_CIRCUIT_DATA`, instead use `cache_get_standard_rec_main_pod_common_circuit_data`
- The library is now using `nightly-2025-07-02`.  Some rust language features are unstable in previous versions.

src/frontend/serialization.rs

@@ -119,7 +119,9 @@ mod tests {
     use super::*;
     use crate::{
         backends::plonky2::{
-            mainpod::Prover, mock::mainpod::MockProver, primitives::ec::schnorr::SecretKey,
+            mainpod::{rec_main_pod_circuit_data, Prover},
+            mock::mainpod::MockProver,
+            primitives::ec::schnorr::SecretKey,
             signedpod::Signer,
         },
         examples::{
@@ -130,7 +132,7 @@ mod tests {
         middleware::{
             self,
             containers::{Array, Dictionary, Set},
-            Params, TypedValue, DEFAULT_VD_SET,
+            Params, TypedValue, DEFAULT_VD_LIST,
         },
     };
 
@@ -300,7 +302,9 @@ mod tests {
             max_input_recursive_pods: 1,
             ..Default::default()
         };
-        let vd_set = &*DEFAULT_VD_SET;
+        let mut vds = DEFAULT_VD_LIST.clone();
+        vds.push(rec_main_pod_circuit_data(&params).1.verifier_only.clone());
+        let vd_set = VDSet::new(params.max_depth_mt_vds, &vds).unwrap();
 
         let (gov_id_builder, pay_stub_builder, sanction_list_builder) =
             zu_kyc_sign_pod_builders(&params);
@@ -312,7 +316,7 @@ mod tests {
         let sanction_list_pod = sanction_list_builder.sign(&signer)?;
         let kyc_builder = zu_kyc_pod_builder(
             &params,
-            vd_set,
+            &vd_set,
             &gov_id_pod,
             &pay_stub_pod,
             &sanction_list_pod,