cipher: replace `StreamCipherCoreWrapper` with a buffering macro?

[newpavlov]

In digest we replaced generic buffering wrapper types with buffering macros. It results in a more convoluted and less transparent code from the developer point of view, but, on the other hand, it results in a simpler API for users, since they see only opaque buffered type with an explicit list of implemented traits.

[tarcieri]

I think it would be good in general to give the same treatment to stream ciphers as well as universal-hash, but IMO it's not as important for these types as it was for digest since they're less user-facing, and I wouldn't block the next release on getting it done.