Implement Benchmarks for modular inverse impl

- Added `Criterion` on dev-dependencies and a
benches config section on Cargo.toml

- Added `benchmarks` folder with `benches.rs` file.

This file contains the benchmarks implementation of
the two distinct modular_inverse implementations.

Results:
```
Modular Inverse/Kalinski Modular inverse
                        time:   [50.105 us 50.438 us 50.819 us]
                        change: [-44.740% -44.195% -43.664%] (p = 0.00 < 0.05)
                        Performance has improved.

Modular Inverse/Standard Mod Inv
                        time:   [82.301 us 82.430 us 82.580 us]
```

To run the benchmarks just run: `cargo bench`.

shamirsecretsharing-rs/Cargo.toml

@@ -9,3 +9,12 @@ rand = "0.6.5"
 num = "0.2.0"
 num-bigint = {version = "0.2.2", features = ["rand"]}
 num-traits = "0.2.8"
+
+[dev-dependencies]
+criterion = "0.2"
+
+# Criterion benchmarks
+[[bench]]
+path = "./benchmarks/benches.rs"
+name = "benches"
+harness = false

shamirsecretsharing-rs/benchmarks/benches.rs

@@ -0,0 +1,38 @@
+#[macro_use]
+extern crate criterion;
+extern crate shamirsecretsharing_rs;
+extern crate num_bigint;
+
+use criterion::{Criterion, Benchmark};
+use shamirsecretsharing_rs::*;
+use num_bigint::BigInt;
+
+use std::str::FromStr;
+
+
+mod mod_inv_benches {
+    use super::*;
+
+    pub fn bench_modular_inv(c: &mut Criterion) {
+
+        let modul1 = BigInt::from_str("7237005577332262213973186563042994240857116359379907606001950938285454250989").unwrap();
+        let d1 = BigInt::from_str("182687704666362864775460604089535377456991567872").unwrap();
+
+        let modul2 = BigInt::from_str("7237005577332262213973186563042994240857116359379907606001950938285454250989").unwrap();
+        let d2 = BigInt::from_str("182687704666362864775460604089535377456991567872").unwrap();
+
+        c.bench(
+            "Modular Inverse",
+            Benchmark::new("Kalinski Modular inverse", move |b| b.iter(|| kalinski_inv(&d1, &modul1)))
+        );
+
+        c.bench(
+            "Modular Inverse",
+            Benchmark::new("Standard Mod Inv", move |b| b.iter(|| mod_inverse(d2.clone(), modul2.clone())))
+        );
+    }
+}
+
+criterion_group!(benches,
+                mod_inv_benches::bench_modular_inv);
+criterion_main!(benches);