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.

Cargo.toml

@@ -2,6 +2,7 @@
 name = "pod2"
 version = "0.1.0"
 edition = "2021"
+build = "build.rs"
 
 [lib]
 name = "pod2"
@@ -19,7 +20,7 @@ env_logger = "0.11"
 lazy_static = "1.5.0"
 thiserror = { version = "2.0.12" }
 # enabled by features:
-plonky2 = { git = "https://github.com/0xPolygonZero/plonky2", optional = true }
+plonky2 = { git = "https://github.com/0xPARC/plonky2.git", rev = "3defd60532c8693cf5e9d2e6a8412c77ca58760f", optional = true }
 serde = "1.0.219"
 serde_json = "1.0.140"
 base64 = "0.22.1"
@@ -32,6 +33,11 @@ rand = "0.8.5"
 hashbrown = { version = "0.14.3", default-features = false, features = ["serde"] }
 pest = "2.8.0"
 pest_derive = "2.8.0"
+directories = { version = "6.0.0", optional = true }
+minicbor-serde = { version = "0.5.0", features = ["std"], optional = true }
+serde_bytes = "0.11"
+serde_arrays = "0.2.0"
+sha2 = { version = "0.10.9" }
 
 # Uncomment for debugging with https://github.com/ed255/plonky2/ at branch `feat/debug`.  The repo directory needs to be checked out next to the pod2 repo directory.
 # [patch."https://github.com/0xPolygonZero/plonky2"]
@@ -42,10 +48,15 @@ pretty_assertions = "1.4.1"
 # Used only for testing JSON Schema generation and validation.
 jsonschema = "0.30.0"
 
+[build-dependencies]
+vergen-gitcl = { version = "1.0.0", features = ["build"] }
+
 [features]
-default = ["backend_plonky2", "zk"]
+default = ["backend_plonky2", "zk", "mem_cache"]
 backend_plonky2 = ["plonky2"]
 zk = []
 metrics = []
 time = []
 examples = []
+disk_cache = ["directories", "minicbor-serde"]
+mem_cache = []