Zoe reveal whether (to what extent) an Offer's want was satisfied

[Chris-Hibbert]

What is the Problem Being Solved?

Zoe's guarantee is that at least one of the user's give and want were satisfied. (With multiples #5422, we will have to ask how many times the want was satisfied). Contracts shouldn't be trusted to answer this question. So Zoe should at least provide a helper that gives a simple answer to the question "Did my trade go through?"

Description of the Design

In addition to userSeat.hasExited(), we could add userSeat.wantSatisfied(). For now, it would be a Promise that resolves to 0 or 1. With Multiples, the number might be greater than 1. The holder of the userSeat would still have to check whether any of the give was returned, as that is orthogonal.

Security Considerations

Doesn't reveal anything that should be hidden.

Test Plan

straightforward. also ensure that the result is reasonable with various forms of exit.