Adding cSHAKE digest

[NIIBE Yutaka]

Hello,

In the task T6637, adding cSHAKE and KMAC is proposed.  I read the
patch, while it works somehow, it is not easy to merge it directly.

Thus, I do implement cSHAKE part, with minimum change.  Attached is my
try.

I plan to take the test vectors for cSHAKE from the patch in T6637 and
add them.
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