API to add data to the shake_context

[oerdnj]

And yet another API request. I think it would be useful to have shake_inject() aliased to hawk_{sign,verify}_adddata() and shake_context typedefed to hawk_context. This would encapsulate all API calls to the hawk_ namespace and you could import the shake implementation only internally.

Also I would like to thank the authors for not making my eyes bleed while reading the source code. The cryptographic code is obfuscated by nature, so an extra care must be taken, so the result is easy to read to the extent possible. You did a really good job there.

Wishing you luck in the NIST PQC Round 2!