Trait for tweakable block ciphers

[newpavlov]

Threefish is an example of tweakable block cipher, thus ideally we need an appropriate trait for it.

One option is to use something like this:

pub trait TweakableBlockCipher {
    type BlockSize: ArrayLength<u8>;
    type TweakSize: ArrayLength<u8>;

    fn encrypt(&self, b: &mut Block<Self::BlockSize>,  t: &Block<Self::TweakSize>);

    fn decrypt(&self, b: &mut Block<Self::BlockSize>, t: &Block<Self::TweakSize>);
}

Additional context can be found in the previous issue.