API Changes

Things that I want to do better for the 1.0.0 release. I'm not about to release that though, I want to iterate on this library for a while. If anybody wants to I'm happy to let somebody have a crack at this issue.

• Move the implementations of Display and FromStr into some internal traits so that they're not exposed in the API. Parsing these library types are implementation details that this library should be abstracting away.
• Make Group unconstructable and add a new method for User and Group that allows easy-ish construction. Along with this, refactor the AllGroups::add_group and AllUsers::add_user so that they just take a User or a Group. This should simplify the API and make it more consistent. Also allows for more efficient use of the library.
  • An alternative to a new function would be a builder pattern, probably using a new type (UserBuilder/GroupBuilder), which would ensure that all their inputs are valid before building a new user. Might be nice to implement construction as User::builder() -> UserBuilder for ergonomic reasons.
  • Alternatively, construct the UserBuilder/GroupBuilder from AllUsers/AllGroups, and cause it to hold a mutable reference to its AllUsers/AllGroups. Consuming the Builder then modifies the All.
• Remove redox_users::Result and just use Result<T, failure::Error> Use thiserror to derive error for the library's error type, and provide more/better context for that error type.
• Implement Debug for AllGroups and fix the implementation for AllUsers. It works right now, but it's not pretty.