refactor: Improve memory efficiency in Nova's abomonation proving system

- Removed the `Clone` trait from `PublicParams` struct in `nova.
- Enhanced doc for memory safety in `PublicParamMemCache` in `mem_cache.rs`
- Replaced cloning commands with `Arc::from_raw` function, resulting in improved memory utilization.

src/proof/nova.rs

@@ -107,7 +107,7 @@ pub type NovaCircuitShape<F> = R1CSWithArity<E1<F>>;
 pub type NovaPublicParams<F, C1> = nova::PublicParams<E1<F>, E2<F>, C1, C2<F>>;
 
 /// A struct that contains public parameters for the Nova proving system.
-#[derive(Clone, Serialize, Deserialize)]
+#[derive(Serialize, Deserialize)]
 #[serde(bound = "")]
 pub struct PublicParams<F, SC: StepCircuit<F>>
 where

src/public_parameters/mem_cache.rs

@@ -40,6 +40,10 @@ pub(crate) static PUBLIC_PARAM_MEM_CACHE: Lazy<PublicParamMemCache> =
     });
 
 impl PublicParamMemCache {
+    /// ## Safety
+    // This assumes:
+    // 1. no one is mutating these params, which is reasonable if all accesses are Lurk processes,
+    // 2. our process only reads this once, otherwise the produced Arc will leak memory.
     fn get_from_disk_cache_or_update_with<
         'a,
         F: CurveCycleEquipped,
@@ -66,7 +70,7 @@ impl PublicParamMemCache {
                     let (pp, rest) =
                         unsafe { decode::<PublicParams<F, C1LEM<'a, F, C>>>(&mut bytes).unwrap() };
                     assert!(rest.is_empty());
-                    Ok(Arc::new(pp.clone())) // this clone is VERY expensive
+                    Ok(unsafe { Arc::from_raw(pp as *const PublicParams<F, C1LEM<'a, F, C>>) })
                 }
                 Err(Error::IO(e)) => {
                     warn!("{e}");