Little key improvements

• Better TestLib literal expressibility support.
• Remove silly typealias JSONAPIDocument.BodyData which pointed to JSONAPIDocument.Body.Data
• Expose initializer for JSONAPIDocument.Body.Data (most useful for testing purposes).
• Add ReversibleTransformer (most useful for testing purposes).
• Add ability to differentiate between EntityType and EntityProxy, the latter of which is intended to allow you to create clientside values that act like EntityType without accidentally calling those values "Entities," which is a word more or less reserved by this library to refer to values that are persisted between the server and client.