Use serde-json for deserialization of `config.json`

Right now we have "proprietary" json-deserializer and an issue to make it fully aligned with `serde-json` or another serialization library.

Instead of maintaining our own implementation it looks like a better way to use `serde-json`.

We already have it as a transitive dependency:

iroha v0.1.0 (/home/humb1t/workspace/soramitsu/iroha/iroha)
...
├── iroha_derive v0.1.0 (/home/humb1t/workspace/soramitsu/iroha/iroha_macro/iroha_derive)
│   ├── iroha_macro v0.1.0 (/home/humb1t/workspace/soramitsu/iroha/iroha_macro)
│   │   └── parity-scale-codec v1.3.0
...
│   │       ├── parity-scale-codec-derive v1.2.0
│   │       │   ├── proc-macro-crate v0.1.4
│   │       │   │   └── toml v0.5.6
│   │       │   │       └── serde v1.0.110
│   │       │   │           └── serde_derive v1.0.110
...
│   │       └── serde v1.0.110 (*)
...
└── ursa v0.3.2
...
    ├── amcl_wrapper v0.1.7
...
    │   ├── serde v1.0.110 (*)
    │   ├── serde_derive v1.0.110 (*)
    │   ├── serde_json v1.0.53
...
    ├── secp256k1 v0.17.2
...
    │   └── serde v1.0.110 (*)
    ├── serde v1.0.110 (*)
...
[dev-dependencies]
├── criterion v0.3.2
...
│   ├── csv v1.1.3
│   │   ├── bstr v0.2.13
...
│   │   │   └── serde v1.0.110 (*)
...
│   │   └── serde v1.0.110 (*)
...
│   ├── serde v1.0.110 (*)
│   ├── serde_derive v1.0.110 (*)
│   ├── serde_json v1.0.53 (*)
│   ├── tinytemplate v1.0.4
│   │   ├── serde v1.0.110 (*)
│   │   └── serde_json v1.0.53 (*)
...

2. Implementation of de\serialization functionality without `serde` hard and error prone.

3. We may use additional data formats like `toml`, `yaml` and other provided by `serde` ecosystem to make it easier for different developers to use Iroha.