sha3: add cShake

[niluxv]

Add cShake, as described in [1] to the sha3 crate (it is a standard way to input a diversifier to shake separate from the normal input).

[1]: https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-185.pdf

[tarcieri]

@newpavlov do you think we should track this in #1?

[newpavlov]

I thought it will be implemented as small helper functions on top of the existing Shake types, but reading the specification a bit more carefully, I guess we can consider it as an additional algorithm. There is an annoying property that if n and s are empty, then cShake is equal to the usual Shake which uses 0x1f suffix for padding, otherwise in addition to prepending to data encoded parameter strings it also uses 0x04 suffix. So assuming that we want to keep streaming capabilities, we will have to keep the padding variant in hasher states.