feat: implement crate halo2_alt with fflonk backend

Cargo.toml

@@ -1,5 +1,6 @@
 [workspace]
 members = [
     "halo2",
+    "halo2_alt",
     "halo2_proofs",
 ]

halo2_alt/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "halo2_alt"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+halo2_proofs = { path = "../halo2_proofs" }
+rand_core = { version = "0.6", default-features = false }
+rayon = "1.8"
+blake2b_simd = "1"
+
+[dev-dependencies]
+rand_chacha = "0.3"
+
+[features]
+sanity-checks = []

halo2_alt/src/backend.rs

@@ -0,0 +1,23 @@
+use crate::protocol::Protocol;
+use halo2_proofs::plonk::Error;
+use rand_core::RngCore;
+use std::fmt::Debug;
+
+pub mod fflonk;
+
+pub trait Circuit<F>: Debug {
+    type WitnessBuf: Debug + Default;
+
+    fn protocol(&self) -> Protocol<F>;
+
+    fn preprocess(&self) -> Result<Vec<Vec<F>>, Error>;
+
+    fn witness(
+        &self,
+        phase: usize,
+        values: &[&[F]],
+        challenges: &[F],
+        buf: &mut Self::WitnessBuf,
+        rng: impl RngCore,
+    ) -> Result<Vec<Vec<F>>, Error>;
+}