`make_promise_contract` instead of executor property queries in the wording

[brycelelbach]

It may make things more readable.

[dhollman]

Do you mean make_promise_contract()?

[brycelelbach]

Yes.

[brycelelbach]

@LeeHowes is this alright?

[LeeHowes]

I don't think it hurts.

[LeeHowes]

Ok, I've been looking into this and I'm not convinced.

We do not currently have a make_cancellable_promise_contract free function. We could - but given the additional complexity of defining the cancellation callback I'm not sure there is value to it. Inside futures code I don't think the free function is very valuable anyway. If we don't have the cancellable contract function, then we need the query anyway as an option for the cancellable form in the text.

In fact, we might want to go the other way and change the precondition of via to define in terms of the query, unless we are assuming that the non-cancellable query is always well-formed if the cancellable one is, in which case the precondition as it is is fine.

[LeeHowes]

PR 123 changes preconditions to be in terms of both queries as an example. If we can assume that only one of those queries is necessary then we can stick with just the non-cancellable version, but for consistency if we are not adding make_cancellable_promise_contract I think using the query makes sense.