Add Kawpow hash

[kralverde]

A little messy (wasn't familiar with c++ or node before this) let me know if you want me to clean it up. The kawpow function takes a header hash, nonce, and block height and returns a 64byte buffer; 32 bytes of main hash and 32 bytes of mix hash

[kralverde]

not sure where the external symbol _keccak_hash is coming from...