Fix init() by removing optional key argument

[mitschabaude]

fixes o1-labs/zkapp-cli#406

This removes the optional zkappKey argument to smartContract.init()

For context, this argument was originally added for the case that you make init() a @method, to prove that it was called and to get the provedState = true property on the zkapp account.

If init() is a @method, it means everybody can call it after deploy (since it doesn't require a signature!). That's bad of course, because we want init() to only be callable once; we don't want everyone to be able to reset the state.

The solution to this is to add a precondition on provedState: to call init(), provedState must be false. Since init() causes provedState to become true, and assuming that the state is henceforth only updated with proofs, nobody will ever be able to call it again.

At the time of introducing init(), the proved state precondition was not available. Therefore, as a workaround, we added an optional privateKey argument to init(), and made it prove that privateKey.toPublicKey() equals the account's address. With this workaround, only the owner of the zkapp private key could call init() (but they could do so as often as they wanted). We no longer need this workaround since the provedState precondition is available now.

Removing the optional argument also solves a bug that occured when you didn't annotate the optional argument as a @method argument, but still passed in the private key. It's not supported to leave out a @method argument, but the previous deploy() behaviour made it extremely easy to accidentally cause such a bug. (see o1-labs/zkapp-cli#406, where the Sudoku zkApp didn't specify the zkappKey argument). In general, optional arguments to a zkApp method are an absolute anti-pattern

[mitschabaude]

Unrelated: this points to latest snarkyjs-bindings with a fix for windows o1-labs/o1js-bindings#19