Switch back to 32 byte proof hash

[ailisp]

As reported by 3rd party, 16 byte increase chance of collision, suggestion was to revert to 32 byte hash.
According to author:
tranvictor/ethashproof#7
It need to be at least 128 bits collision resistance, which is 256 bits to SHA2 or 32 bytes

[MaksymZavershynskyi]

This is quite challenging, and is blocked by #150 . Setting estimate to 3 assuming #150 is done.

[Ericson2314]

You could try reverting https://github.com/tranvictor/ethashproof/pull/5/files as a stop-gap

[k06a]

Collision resistance for looking for existing hash preimage is still 2^128, not 2^64 – it should be enough.