Nice API to set state on other accounts

[mitschabaude]

Motivation

We have nice APIs to do most things on accounts other than our own zkapp, like

let update = AccountUpdate.create(address);

// get the other account's balance
let b = update.account.balance.getAndRequireEquals();

// set the other account's permissions
update.account.permissions.set(Permissions.default());

However, we don't have anything for setting state. You can only set state by modifying the raw account update. This is not documented and from experience, many users don't discover this API on their own:

update.body.update.appState[0].isSome = Bool(true);
update.body.update.appState[0].value = Field(1234);

Historically, the inconsistency originated because we designed the state setting API for zkapps around a custom namespace that you define along with the SmartContract, i.e. @state x and this.x.set().

Proposed API

If we set state on other accounts, we can't assume knowing names for state fields. But we can still design something that's nice to use, e.g.

// set individual field elements by providing their index
update.account.state.set(0, Field(1_000));

// set one or many fields by providing a start index and a type
update.account.state.set(1, PublicKey, myAddress);

The above should be reasonably easy to use. But we could make it even more similar to the familiar API by allowing to declare a zkapp-like state layout inline. E.g.

let state = State.withLayout(update, { balance: UInt64, address: PublicKey });
state.balance.set(UInt64.from(1_000));
state.address.set(myAddress);

Apart from set(), we also want preconditions:

let currentBalanceRaw: Field = update.account.state.getAndRequireEquals(0);
let currentBalance: UInt64 = update.account.state.getAndRequireEquals(0, UInt64);

let state = State.withLayout(update, { balance: UInt64, address: PublicKey });
let currentAddress: PublicKey = state.address.getAndRequireEquals()