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

(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]
```

- current:
```
add                     time:   [317.34 ns 317.44 ns 317.54 ns]
mul_scalar              time:   [131.05 us 131.28 us 131.58 us]
```

Which is `168x` improvement for `add`, and `923x` improvement for `mul_scalar`.