Uniform returned values for API functions that produce tx (comments welcome)

[jkrivine]

Proposal, all API functions that produce a signed transaction should return immediately with a record of the form: {ret:Promise<human readable returned values>, receipt: Promise<ethers.TransactionResponse>}

Usage would be something like:

const {ret:values} = (await) api.f(...);
const {x:number, y: boolean} = await values;