Implement EIP-2681: Limit account nonce to 2^64-1

[axic]

Fixes #813. Please note that this applies from genesis, and geth implements it now.

I tried to implement this and I think I found the correct place for the external transaction validation. However I have no good idea where to describe the rules for CREATE/CREATE2.

[axic]

Perhaps the "64-bit" mention here is not entirely correct, as it is serialised via RLP, so it has no fixed size. However since the recipient field below states "160-bit address" I decided to include it.

Potentially the more clear way is to include these restrictions in the validation sections, which means the address should be just a scalar here, and a new rule would need to be added.

[acoglio]

Equations (13) and (17) say that the nonce is a 256-bit word. Should they be changed too?

[axic]

Good point @acoglio.

I think (13) is worth changing as that already mentions "types" (such as B_32 vs N_256) and doesn't deal with RLP.

However not sure about (17). That seems to deal with RLP encoded data and is less strict about types. It is also missing T_t which should be N_160. Never mind, it is described in (19). In this case I'll update both (13) and (17).