The Chain Specification

By default, when simply running CKB, CKB will connect to the official public Nervos network.

In order to run a chain different to the official public one, with a config file specifying chain = "path" under [ckb].

Because the limitation of toml library, we must put nested config struct in the tail to make it serializable, details https://docs.rs/toml/0.5.0/toml/ser/index.html