cipher: remove padded methods from BlockCipherEncrypt/Decrypt traits

[newpavlov]

The methods were intended to be used for the ECB mode, but they are hard to discover in practice for inexperienced users, so I plan to introduce a separate crate for the mode.

Closes #2129

[newpavlov]

@tarcieri
WDYT about the remaining methods? While they are convenient and probably expected by users, they are problematic since relying on them results in potential backend branching on each method call, which can be quite sub-optimal. In particular, a number of AEAD crates suffer from that.

I wonder if we should keep only encrypt/decrypt_with_backend methods to force users to properly handle block cipher implementations with multiple backends selected at runtime.

[tarcieri]

I'll say it's a little strange our block cipher traits don't provide the API typically provided by block ciphers whatsoever, but only somewhat arcane performance-oriented APIs