Don't increment nonces on accounts that are the fee payer

[mitschabaude]

• closes Don't increment nonces on accounts that are the fee payer #271
• tweak the implementation of AccountUpdate.createSigned, to use create and requireSignature(). Deprecate the API where a private key is passed to createSigned() -- we want to move away from private keys in transactions. Instead, add another function signature that takes a public key.
• breaking change to the circuit used in requireSignature() -- this will break many deployed contracts and thus require a minor version bump

[MartinMinkov]

Awesome change! Everything checks out on my end :D