Groestl perfomance

[newpavlov]

Current implementation of Groestl is quite slow (<1 MB/s) and can be significantly improved.

Various performance improvement techniques can be found in the "Grøstl Implementation Guide". List of the fastest implementations can be found here.

[newpavlov]

In #217 performance was significantly improved and now it's on an acceptable level of ~100 MB/s. There is still a lot of room for improvement, especially via utilization of AES-NI and SIMD instructions.