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.

build.rs

@@ -0,0 +1,21 @@
+#[cfg(feature = "mem_cache")]
+fn main() {}
+
+#[cfg(feature = "disk_cache")]
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    use vergen_gitcl::{Emitter, GitclBuilder};
+    // Example of injected vars:
+    // cargo:rustc-env=VERGEN_GIT_BRANCH=master
+    // cargo:rustc-env=VERGEN_GIT_COMMIT_AUTHOR_EMAIL=emitter@vergen.com
+    // cargo:rustc-env=VERGEN_GIT_COMMIT_AUTHOR_NAME=Jason Ozias
+    // cargo:rustc-env=VERGEN_GIT_COMMIT_COUNT=44
+    // cargo:rustc-env=VERGEN_GIT_COMMIT_DATE=2024-01-30
+    // cargo:rustc-env=VERGEN_GIT_COMMIT_MESSAGE=depsup
+    // cargo:rustc-env=VERGEN_GIT_COMMIT_TIMESTAMP=2024-01-30T21:43:43.000000000Z
+    // cargo:rustc-env=VERGEN_GIT_DESCRIBE=0.1.0-beta.1-15-g728e25c
+    // cargo:rustc-env=VERGEN_GIT_SHA=728e25ca5bb7edbbc505f12b28c66b2b27883cf1
+    let gitcl = GitclBuilder::all_git()?;
+    Emitter::default().add_instructions(&gitcl)?.emit()?;
+
+    Ok(())
+}