digest: implement stream cipher trait for XOF readers

[newpavlov]

Probably depends on introduction of a block-level trait for (synchronous) stream ciphers. After that a feature-gated blanket implementation will look somewhat like this:

impl<R: XofReaderCore> XofReader for XofReaderCoreWrapper<R> { .. }
impl<R: StreamCipherCore> StreamCipher for XofReaderCoreWrapper<R> { .. }

It would mean that core XOF reader types would have to implement both XofReaderCore and StreamCipherCore and there will be a bit of code duplication with a wrapper defined in cipher, but it should not be a big problem.