Add benchmarks (49x improvement with ff)

Tested on a Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz, with 16GB of RAM.

- Old (using nnum-bigint):
hash           time:   [5.9258 ms 5.9407 ms 5.9587 ms]

- New (using ff):
hash           time:   [120.12 us 121.08 us 122.30 us]

benches/bench_poseidon_hash.rs

@@ -0,0 +1,30 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+
+extern crate rand;
+#[macro_use]
+extern crate ff;
+use ff::*;
+
+use poseidon_rs::Poseidon;
+
+fn criterion_benchmark(c: &mut Criterion) {
+    let b1: poseidon_rs::Fr = poseidon_rs::Fr::from_str(
+        "12242166908188651009877250812424843524687801523336557272219921456462821518061",
+    )
+    .unwrap();
+    let b2: poseidon_rs::Fr = poseidon_rs::Fr::from_str(
+        "12242166908188651009877250812424843524687801523336557272219921456462821518061",
+    )
+    .unwrap();
+    let mut big_arr: Vec<poseidon_rs::Fr> = Vec::new();
+    big_arr.push(b1.clone());
+    big_arr.push(b2.clone());
+    let poseidon = Poseidon::new();
+
+    c.bench_function("hash", |b| {
+        b.iter(|| poseidon.hash(big_arr.clone()).unwrap())
+    });
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);