Handle return values from `addCommand` better

[Arachnid]

Ideally you would be able to transparently handle functions that return 0, 1, or more return values intuitively. Eg, a function that returns one argument would look like:

const ret = planner.addCommand(contract.func(...));

and a function that returns two would look like:

const [ret1, ret2] = planner.addCommand(contract.func(...));

At the moment, however, typescript forces you to explicitly cast the result, or it complains it doesn't know which your function returns. We should fix this somehow.

[Arachnid]

Fixed by only allowing one return value.