Refactor towards Codable, Identifiable and value types

[fwcd]

This big PR that will fix #2, refactor the structures towards a more modern, value-oriented design and more.

• [ ] Migrate to Codable (especially Decodable) and Identifiable (in progress)
• [ ] Define gateway event structures instead of using the weakly typed payload objects in DiscordClient
• [ ] Migrate to RawRepresentables over enums where possible
• [ ] Remove client references from support structures (making them plain old value types)
• [ ] Rename types, dropping the Discord prefix to be more swifty (consumers of the library can still prefix the types with Discord. to disambiguate)
• [ ] Update the unit tests

[fwcd]

Whoops, I meant to open the PR in my fork for now. Sorry!