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/backends/plonky2/circuits/mainpod.rs

@@ -14,6 +14,7 @@ use plonky2::{
     },
     plonk::config::AlgebraicHasher,
 };
+use serde::{Deserialize, Serialize};
 
 use crate::{
     backends::plonky2::{
@@ -28,7 +29,7 @@ use crate::{
             },
             signedpod::{verify_signed_pod_circuit, SignedPodVerifyTarget},
         },
-        emptypod::{EmptyPod, STANDARD_EMPTY_POD_DATA},
+        emptypod::{cache_get_standard_empty_pod_circuit_data, EmptyPod},
         error::Result,
         mainpod::{self, pad_statement},
         primitives::merkletree::{
@@ -1303,6 +1304,7 @@ fn verify_main_pod_circuit(
     Ok(id)
 }
 
+#[derive(Clone, Serialize, Deserialize)]
 pub struct MainPodVerifyTarget {
     params: Params,
     vds_root: HashOutTarget,
@@ -1427,9 +1429,13 @@ impl InnerCircuit for MainPodVerifyTarget {
             self.vd_mt_proofs[i].set_targets(pw, true, vd_mt_proof)?;
         }
         // the rest of vd_mt_proofs set them to the empty_pod vd_mt_proof
-        let vd_emptypod_mt_proof = input
-            .vds_set
-            .get_vds_proofs(&[STANDARD_EMPTY_POD_DATA.1.verifier_only.clone()])?;
+        let vd_emptypod_mt_proof =
+            input
+                .vds_set
+                .get_vds_proofs(&[cache_get_standard_empty_pod_circuit_data()
+                    .1
+                    .verifier_only
+                    .clone()])?;
         let vd_emptypod_mt_proof = vd_emptypod_mt_proof[0].clone();
         for i in input.vd_mt_proofs.len()..self.vd_mt_proofs.len() {
             self.vd_mt_proofs[i].set_targets(pw, true, &vd_emptypod_mt_proof)?;