Make fields of struct Signature public

- Update aarch64 feature related code
- Update benchmarks to last lib updates
- Apply Clippy fixes

Update rust to 2021 and deps

Update to last Poseidon version (which includes the changes of the reference implementation from
26ddaa91db)

Compatible with update at circomlib
(cf853c1cc9)

- Update EdDSA to last circomlib version
	- Compatible with circomlib version 0.3.0 at
	  5935ac69df
	- Use blake hash precandidate (non-blake2b) for circomlib
	  compatibility
	- Add circomlib testvector
- Change PrivateKey from BigInt to [u8;32], which lead to faster
signature computation:
	(Benchmarks on a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with
	32 GB of RAM)
	- Old:
	    ```
	    sign                    time:   [559.84 us 568.41 us 576.26 us]
	    verify                  time:   [376.59 us 376.68 us 376.78 us]
	    ```
	- New:
	    ```
	    sign                    time:   [383.85 us 390.31 us 396.44 us]
	    verify                  time:   [371.33 us 376.06 us 381.19 us]
	    ```
- crate version v0.0.5

- update poseidon-rs to v0.0.5
- remove TravisCI and add GithubAction for tests
- remove mimc-rs support

Benchmarks (On a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with 32 GB of RAM):
- Old:
```
sign                    time:   [953.50 us 953.73 us 953.98 us]
verify                  time:   [832.24 us 832.38 us 832.52 us]
```

- New:
```
sign                    time:   [559.84 us 568.41 us 576.26 us]
verify                  time:   [376.59 us 376.68 us 376.78 us]
```

Add [ff](https://github.com/matter-labs/ff) for internal finite field operations, and add `add-2008-bbjlp` for point addition.

Benchmarks (On a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with 32 GB of RAM):
```
- before:
add               time:   [53.447 us 53.467 us 53.492 us]
mul_scalar        time:   [121.19 ms 121.22 ms 121.25 ms]
sign              time:   [383.01 ms 384.46 ms 385.98 ms]
verify            time:   [250.56 ms 251.46 ms 252.43 ms]

- current:
add               time:   [317.34 ns 317.44 ns 317.54 ns]
mul_scalar        time:   [131.05 us 131.28 us 131.58 us]
sign              time:   [973.38 us 973.83 us 974.41 us]
verify            time:   [835.34 us 839.94 us 845.29 us]
```

add: `168x` improvement
mul_scalar: `923x` improvement
sign: `394x` improvement
verify: `300x` improvement

Still needs a refactor.

(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