Limit account nonce to be between `0` and `2^64-1`.
## Motivation
This is motivated by Eth1.x / Stateless Ethereum discussions, more specifically discussion around the ["witness format"](https://github.com/ethereum/stateless-ethereum-specs).
Introducing a restriction would allow storing the nonce in a more optimised way.
Additionally it could prove beneficial to transaction formats, where some improvements are potentially sought by at least three other proposals.
Lastly this facilitates a minor optimisation in clients, because the nonce no longer needs to be kept as a 256-bit number.
## Specification
If `block.number >= FORK_BLOCK` introduce two new restrictions:
1. Consider any transaction invalid, where the nonce exceeds `2^64-1`.
2. The `CREATE` instruction to abort with an exceptional halt, where the account nonce is `2^64-1`.
## Rationale
1. It is unlikely for any nonce to reach or exceed the proposed limit. If one would want to reach that limit via external transactions, it would cost at least `21000 * (2^64-1) = 387_381_625_547_900_583_915_000` gas.
2. It must be noted that in the past, in the Morden testnet, each new account had a starting nonce of `2^20` in order to differentiate transactions from mainnet transactions.
