feat(rln): add rln zkey based circuit abstraction

Different from CircomConfig etc

rln/src/circuit.rs

@@ -0,0 +1,28 @@
+/// Adapted from semaphore-rs
+use ark_bn254::{Bn254, Fr};
+use ark_circom::{read_zkey, WitnessCalculator};
+use ark_groth16::ProvingKey;
+use ark_relations::r1cs::ConstraintMatrices;
+use core::include_bytes;
+use once_cell::sync::Lazy;
+use std::io::{Cursor, Write};
+use tempfile::NamedTempFile;
+
+const ZKEY_BYTES: &[u8] = include_bytes!("../resources/rln_final.zkey");
+const WASM: &[u8] = include_bytes!("../resources/semaphore.wasm");
+
+pub static ZKEY: Lazy<(ProvingKey<Bn254>, ConstraintMatrices<Fr>)> = Lazy::new(|| {
+    let mut reader = Cursor::new(ZKEY_BYTES);
+    read_zkey(&mut reader).expect("zkey should be valid")
+});
+
+pub static WITNESS_CALCULATOR: Lazy<WitnessCalculator> = Lazy::new(|| {
+    // HACK: ark-circom requires a file, so we make one!
+    let mut tmpfile = NamedTempFile::new().expect("Failed to create temp file");
+    let written = tmpfile.write(WASM).expect("Failed to write to temp file");
+    assert_eq!(written, WASM.len());
+    let path = tmpfile.into_temp_path();
+    let result = WitnessCalculator::new(&path).expect("Failed to create witness calculator");
+    path.close().expect("Could not remove tempfile");
+    result
+});