Redesign Symmetric types

[kyay10]

The key trick is to use SymmetricEncryptionAlgorithm to hold the type parameters (i.e. Auth and Nonce). Then, the sealed hierarchies no longer rely on interfaces, instead, they provide extensions that when over the relevant inheritors, and use absurd when an inheritor cannot possibly be called on this method.

[kyay10]

There's one test in ValidationApiTest that seems broken due to a certificate expiry. I get the failure without my changes, so it's not my code that caused it lol