[WIP] Unification with Codable

[Anviking]

This is an experiment to see what happens when we remove all existing functionality and rely on Swift 4's Codable instead.

• Remove all existing functionality
• Create a few overloads (this initial implementation is likely suboptimal complete nonsense though)
• Update tests
• Investigate how different use-cases of e.g DynamicDecodable would be translated (subclassing JSONDecoder?)