Implement new identity management

The main data structure responsible for complete identity management, identity::Complete, contains the Author information, secret key, public key & other associated metadata. Various functions are implemented on the aforementioned struct that handle identity creation, modification, deletion & migration from the old format. These functions are then leveraged in pijul/src/commands/identity.rs to expose the functionality to the user.

Created by  finchie  on August 16, 2022
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