Update methods to more efficient usage of ff

On a Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz, with 16GB of RAM:
- Old (using num-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]

On a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with 32 GB of RAM:
- Old (using num-bigint):
hash  time:   [4.1192 ms 4.1318 ms 4.1461 ms]
- In the previous commit (using ff):
hash  time:   [91.394 us 91.430 us 91.476 us]
- In this commit (using ff):
hash  time:   [85.517 us 85.545 us 85.574 us]

benches/bench_poseidon_hash.rs

@@ -1,4 +1,4 @@
-use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use criterion::{criterion_group, criterion_main, Criterion};
 
 extern crate rand;
 #[macro_use]