Add a State::set_nonce_at method

[tdelabro]

When using a transactional state you end up with state changes that include an address nonce going from value n to value n + x, where x can be a number between n and a lot.

The State trait only exposes the increment_nonce method to interact with this specific storage. This means that if I want to apply my state changes to my actual storage I will have to keep calling increment_nonce inside a loop until it reaches the correct value. This is a very bad API. A set_nonce_at method should be exposed in order to set the new value in a single write.