game security

[LokartBC]

Hi, I found 2 security issues that need improvment:

• adding private salt input to board hashing to prevent attacker from computing the hash
• ship positions must be verified to be sure boats don't overlap which would be cheating

[lhoste-bell]

I found an additional issue: currently the positions of the ships can be modified during gameplay by the prover.
It is required to enforce x/y/o coordinates to 2^4 bits such that the prover cannot manipulate input data which still matches the hash value.