Break up config::load, add internal `validated` module.
Break up config::load, add internal `validated` module.

- Split `config::load` into its constituent pieces to make it easier to deal with.
- Replace `ValidatedUrl` standalone type with `validated::Url` type in new
  `validated` module, exposing `new` and `value` methods to allow
  consumers to safely create and retrieve URL values.

chriskrycho/lightning-rs,chriskrycho/lightning-rs