Rejigger the requires in test_helper.
Shift Signers API to have verify and verify\!
Move openssl require to helper.
Refactor encryptor to use smaller methods.
Push up tampered check logic.

Refactor `generate_auth_tag` to avoid calling `generate_digest`.
This allows us to put the `untampered?` logic in `AbstractBase` and override in `Encryptor`.
Extract auth tag generation to class.
Extract encryption/decryption to a class.
Inline `Digestable` and `SecureComparable` concerns.
Push up some parsing. Introduce before_load hook method.
Add valid? method to AbstractBase.