Version 0.2.0: Beta

There is lots of internal changes – mostly this should result in changes like writing Message<Encrypted<Version2>> as Message<Version2.Local> and SymmetricKey<Version2> as Version2.SymmetricKey.

To see examples of migration, it may be useful to review this diff, and perhaps the PR #2 also if there is interest into why these changes have occurred.

It's also no longer possible to try to use the "Public" purpose API in version 1 (previously this would throw exceptions or invoke a fatal error since this is not supported – it now just doesn't exist). We now no longer need to create "diverse" types by projecting a type argument to an enum state and alternating behaviour accordingly – instead where different types are required we have used associated types.