Allow yielding a chain of then on Task

[SebastienGllmt]

Motivation

There are cases where you need to do the following steps:

1. Create a Task (using run)
2. Do some computation on the Task using then
3. yield the result of the task

example that shows how this can be used to:

• implement Add equivalent to shared Promise to FAQ #946 without requiring to use async/await explicitly anywhere
• easily chain things without having to awkwardly wrap results with call as in Add equivalent to Promise.then to FAQ #944

class Foo {
    task: Future<number>;
    
    constructor() {
       this.task = run(() => ...).then(val => val+1);
    }

    *doWork() {
        yield* this.task;
    }
}

This was a bit tedious to do previously (see #944), and this PR simplifies this case a bit by making that calling then on a Task returns a Future (which you can yield) instead of just a Promise

Approach & Alternate Designs

You can find some background in #944 and inline comments in this PR

TODOs and Open Questions

• Do we want to make creating Futures more easier in general? This has a chance of overlapping conceptually with the pipe concept that was recently removed, and also overlaps with the idea of call (although call returns an Operation instead of a Future)
• Is there a way to make that then returns a Task instead of a Future? I tried this initially, but ran into some complications: Add equivalent to Promise.then to FAQ #944 (comment)

[SebastienGllmt]

note: the code here for handling the abort future is mostly the same as the then in the parent Task

This is why I think maybe it's better if we have a more general way to create a Future that is exposed somewhere that is used in both these places

There is, to some extent, a conceptual overlap between what this block of code is trying to do and what call is doing. That is to say, if call returned a Future instead of an Operation, using it here could also be an option

[SebastienGllmt]

Note: these three are copy-pasted from the definition of Promise, but have the return type changed from Promise → Future

[SebastienGllmt]

Note: there is one very subtle change introduced by this PR: the result of these is no longer a Promise and, instead, is now a PromiseLike. In practice, this should work everywhere (the goal of PromiseLike is that they're meant to be supported by everything magically), but it is possible to write code that only works on Promises and not PromiseLike (ex: any code that uses instanceof Promise)

[cowboyd]

I'm wondering about the corner cases here.

1. Will the returned promises be lazy? I.e.
2. What if the return value is an operation vs a promise vs a regular value?

I.e. what would be the expectation around this:

run(fn).then(function*(value) { return value + 1 })

[SebastienGllmt]

1. The laziness is not changed by this PR. Nothing happens (lazy) until you use await just like before this PR
2. Currently, the behavior is the same as promises in generally. That is to say,
   1. if you return a value, it stays a value
   2. if you return a promise, it gets flattened to a value (.then(() => Promise.resolve(3)) becomes just 3)
   3. If you return a generator, it doesn't get flattened (it gets treated as case 1 and just returns a generator)

You could say that may we would like then to flatten generators as well for usability reasons, but this is not possible because the type definition of then comes from Promise (we cannot just arbitrarily change it). You could introduce a new function in the Future interface that flattens both promises in generators if we want

[cowboyd]

This should be addressed by #1065 and thefrontside/effectionx#112

Please let us know if there is anything we missed.