Nettle: Unified cipher16.H and cipher.H.
Some cleanups to reduce code duplication.
Introduces a new intermediate class Nettle.BlockCipher
intended to hold stuff common to all block ciphers.
Renamed Nettle.Cipher16 to Nettle.BlockCipher16 for clarity.
Nettle: Moved GCM to a Cipher submodule.
The various tastes of GCM now follow the AEAD API properly.
Crypto.GCM is no more, instead there are Crypto.AES.GCM,
Also updates the SSL code accordingly.