Update sign&verify (poseidon) to last optimization

(On a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with 32 GB of RAM)

- before:
```
sign_poseidon           time:   [383.01 ms 384.46 ms 385.98 ms]
verify_poseidon         time:   [250.56 ms 251.46 ms 252.43 ms]
```

- current:
```
sign_poseidon           time:   [973.38 us 973.83 us 974.41 us]
verify_poseidon         time:   [835.34 us 839.94 us 845.29 us]
```

sign_poseidon: `394x` improvement
verify_poseidon: `300x` improvement

Cargo.toml

@@ -11,6 +11,7 @@ readme = "README.md"
 [dependencies]
 ff = {package="ff_ce" , version="0.11", features = ["derive"]}
 rand = "0.4"
+rand6 = {package="rand", version="0.6.5"}
 num = "0.2.0"
 num-bigint = {version = "0.2.2", features = ["rand"]}
 num-traits = "0.2.8"
@@ -19,7 +20,7 @@ generic-array = "0.13.2"
 tiny-keccak = "1.5"
 rustc-hex = "1.0.0"
 mimc-rs = "0.0.2"
-poseidon-rs = "0.0.1"
+poseidon-rs = "0.0.3"
 arrayref = "0.3.5"
 lazy_static = "1.4.0"