Implement SHAKE128 and SHAKE256 functions

[hauleth]

[hauleth]

There is notable issue with SHAKE128 and SHAKE256 functions implementation. It's output size depends on variable which make Digest::output_bits dependent on internal state. Main problem here is that this would require using Vec in HMAC implementation and it would be impossible to replace it with static array when associated constants lands. Currently there is no RFC to provide template constants that could solve this problem so discussion remains.