Add benchmark

On a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with 32 GB of RAM:
hash	time:   [927.61 us 927.78 us 927.98 us]

benches/bench_mimc_hash.rs

@@ -0,0 +1,27 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+
+use num_bigint::BigInt;
+
+use mimc_rs::Mimc7;
+
+fn criterion_benchmark(c: &mut Criterion) {
+    let b1: BigInt = BigInt::parse_bytes(
+        b"12242166908188651009877250812424843524687801523336557272219921456462821518061",
+        10,
+    )
+    .unwrap();
+    let b2: BigInt = BigInt::parse_bytes(
+        b"12242166908188651009877250812424843524687801523336557272219921456462821518061",
+        10,
+    )
+    .unwrap();
+    let mut big_arr: Vec<BigInt> = Vec::new();
+    big_arr.push(b1.clone());
+    big_arr.push(b2.clone());
+    let mimc7 = Mimc7::new();
+
+    c.bench_function("hash", |b| b.iter(|| mimc7.hash(big_arr.clone()).unwrap()));
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);