[utils] improve serialization when serializing to json

[mimoo]

closes #1037

here's an example of a verifier index

{
  "domain": "20000000000000000500000020000000000000000000000000000000000000000000000000000000000000000100009865c763bc52910229346434210000000000000000000000000000003ecc789b927663a38fbcb7ce3e39a3ae3bed330856d19af2dae1f216c60d38c30c87a189965e1f4df0bafbc05f37cdfcc8edd344b81f32ce5c410fa94ba5bf2f1467666666f8797870713085d064704fa799999999999999999999999999999919",
  "max_poly_size": 32,
  "public": 5,
  "prev_challenges": 0,
  "sigma_comm": [
    {
      "unshifted": [
        "26fd31d1824baae274323cc4379bbefb51f445f0aef6a630b24afbf79f34c92600"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "31f5be1c7f7a59d456eff1fb8e08957640bd59e1f33535ce58bf4d4b5080af3580"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "d9cb6710fe7cd16ce5b4f87ea9800d06d58a182ade0b7d950b8579822fc8fd1c80"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "799e5fd806bc420e4e6a0073c36178b79a6534a58886d1cbf40e808707ba2a2600"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "6d43561d77acd27c23df7106fee13646f79d9daa5c0c80b582236c049d75510c80"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "e31db40a7724332968bf23178c6867120c03304c2fddf04c611f858f169b811800"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "f0eda702ac7c8224ad278573660af582a62ef552a8136a95a06f967001b9d81280"
      ],
      "shifted": null
    }
  ],
  "coefficients_comm": [
    {
      "unshifted": [
        "8dcb26c25c70a937bf06aabd97294fd668192a7d03dc6fe4334be183628b6f2c80"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "2e94340fae7b22dd723eced300be207afe0bb78a1a3b3241a32282c0da8c632a00"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "adb98835474a18cb434b82b03224e0b59849971a17d935b9bb564f22299ac73200"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "000000000000000000000000000000000000000000000000000000000000000040"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "fb9defe1f6185e95cf0db3111d10f3ffdccd5b8c66b009088eeb77a95971412380"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "5ce86c3af1501bd9b2f52cf4968348404ba87f62769a098a6f1fe6a6f946501c80"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "000000000000000000000000000000000000000000000000000000000000000040"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "adb98835474a18cb434b82b03224e0b59849971a17d935b9bb564f22299ac73200"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "b0a92faff6d8822367f6fe3317ee736cc4c3e94ac889e9325305088619fff10c80"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "8c07920f03cdc6167becbf317cd5813e0e1c1a6687288178808e3bf1b0818e0f00"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "000000000000000000000000000000000000000000000000000000000000000040"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "000000000000000000000000000000000000000000000000000000000000000040"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "000000000000000000000000000000000000000000000000000000000000000040"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "000000000000000000000000000000000000000000000000000000000000000040"
      ],
      "shifted": null
    },
    {
      "unshifted": [
        "000000000000000000000000000000000000000000000000000000000000000040"
      ],
      "shifted": null
    }
  ],
  "generic_comm": {
    "unshifted": [
      "08d265f081130655eaec85eed48165efe8ffec4a66da6149d1fb3adb200d261280"
    ],
    "shifted": null
  },
  "psm_comm": {
    "unshifted": [
      "012226265bceb2e5a8c78be27579a29c3636787563f2b4aa99c901633860200980"
    ],
    "shifted": null
  },
  "complete_add_comm": {
    "unshifted": [
      "000000000000000000000000000000000000000000000000000000000000000040"
    ],
    "shifted": null
  },
  "mul_comm": {
    "unshifted": [
      "000000000000000000000000000000000000000000000000000000000000000040"
    ],
    "shifted": null
  },
  "emul_comm": {
    "unshifted": [
      "000000000000000000000000000000000000000000000000000000000000000040"
    ],
    "shifted": null
  },
  "endomul_scalar_comm": {
    "unshifted": [
      "000000000000000000000000000000000000000000000000000000000000000040"
    ],
    "shifted": null
  },
  "range_check0_comm": null,
  "range_check1_comm": null,
  "foreign_field_add_comm": null,
  "foreign_field_mul_comm": null,
  "xor_comm": null,
  "rot_comm": null,
  "shift": [
    "0100000000000000000000000000000000000000000000000000000000000000",
    "e3a214e91334d0caf1eb85df5bd7524d73d5eb7aaf742a7eb2d40bfdc8cdb900",
    "5a696526fa309c412c10e86604c3c0ad2cd9443dd85b823203721281cfbf3300",
    "11848e2c0bf18b0e9f7c8c34db4621b6c75afaa1e32d0f96164e9529bbf48700",
    "9358be9d3ef26bb05bbaaba8c3262e0bb65d36479694d15ef16c9f3b3771ec00",
    "106d491c728c240cf04564b85c586b4072a1f1641fd3684c08554ccc9791f300",
    "a3ee5991230c1903d2dfc543013c3567e0839a29a4e6d212eedb999703ddb800"
  ],
  "lookup_index": null
}

the reason why the domain looks weird in the example above is that the type is a CanonicalSerialize:

#[derive(Copy, Clone, Hash, Eq, PartialEq, CanonicalSerialize, CanonicalDeserialize)]
pub struct Radix2EvaluationDomain<F: FftField> {
    /// The size of the domain.
    pub size: u64,
    /// `log_2(self.size)`.
    pub log_size_of_group: u32,
    /// Size of the domain as a field element.
    pub size_as_field_element: F,
    /// Inverse of the size in the field.
    pub size_inv: F,
    /// A generator of the subgroup.
    pub group_gen: F,
    /// Inverse of the generator of the subgroup.
    pub group_gen_inv: F,
    /// Multiplicative generator of the finite field.
    pub generator_inv: F,
}

as such, when we serialize it, we first serialize that in bytes (via our serialization helper in utils/) and then serialize that again. Unfortunately, since arkworks doesn't support serde, this is what we get.

[mimoo]

ping @jspada or @querolita for review

[jspada]

Nice and simple - looks good to me.