refactor(mithril-stm): refactor and stabilize the SNARK setups

mithril-stm/Cargo.toml

@@ -87,6 +87,7 @@ rug = { version = "1.30.0", optional = true }
 [dev-dependencies]
 blake2b_simd = "1.0.4"
 criterion = { version = "0.8.2", features = ["html_reports"] }
+fs2 = "0.4.3"
 httpmock = "0.8.3"
 num-bigint = "0.4.6"
 num-rational = "0.4.2"

mithril-stm/src/circuits/circuit_key_generator.rs

@@ -0,0 +1,25 @@
+// Lets each circuit derive its own verifying and proving keys from an SRS, so the verification key
+// provider can compute keys on a cache miss without a separate generator type.
+use midnight_curves::Bls12;
+use midnight_proofs::poly::kzg::params::ParamsKZG;
+
+use crate::{
+    StmResult,
+    codec::{TryFromBytes, TryToBytes},
+};
+
+/// A circuit that can generate its verifying and proving keys from a sufficiently sized SRS.
+pub(crate) trait CircuitKeyGenerator {
+    /// Verifying key; round-trips through bytes and is cloneable for caching.
+    type VerifyingKey: TryFromBytes + TryToBytes + Clone;
+    /// Proving key; round-trips through bytes.
+    type ProvingKey: TryFromBytes + TryToBytes;
+
+    /// Derives the verifying and proving keys from a sufficiently sized `srs`: it uses `srs` directly
+    /// when already at the circuit's relation degree, and otherwise downsizes a clone to that degree,
+    /// never mutating the caller's SRS.
+    fn generate_key_pair(
+        &self,
+        srs: &ParamsKZG<Bls12>,
+    ) -> StmResult<(Self::VerifyingKey, Self::ProvingKey)>;
+}