SEC-36 Block header nonce overflow

[Gustav-Simonsson]

The block header nonce field is a byte slice then converted into a uint64 when used in ethash.

If the block hash is taken over the entire byte slice which could be longer than 8 bytes, then a different block hash can be generated but the PoW nonce would appear the same in ethash verification.

This makes it possible for a malicious miner to reuse a nonce in an uncle header.

Fixed in f486c0a