New block-modes API design

[jethrogb]

(from https://github.com/RustCrypto/block-ciphers/pull/227#issuecomment-783074558)

• At the type system, support modes requiring only an block encryption function (as opposed to both encryption and decryption)
• bring back separate types for encryptors and decryptors
• splitting the trait into two parts: core which works only on blocks (and thus padding-independent) and a buffered wrapper on top of it which would work with slices and will be generic over padding.

[jethrogb]

Additional thoughts:

• OFB should be a stream cipher abstraction (like CTR)
• CFB should support a more streaming-like byte-based API (padding is not necessary for decrypting ciphertext of non-block-size-divisible length, and you don't need to encrypt the next block until you have the entire block)

Edit: actually I see these are implemented separately in the stream-ciphers repo 🤦