Singleton observables

[hntrl]

Introduces SingletonAsyncObservable; a utility class for observables that lets you access the value emitted by observables that emit one (and only one) value (like the observables returned from reduce(), count(), etc.) using native await syntax.

This makes the consumption of these single value operators a little bit more readable. For instance:

const obs = AsyncObservable.from([1, 2, 3]);
const singleton = obs.pipe(first());

// instead of this:
let firstValue: number | undefined;
await obs.subscribe((value) => {
  firstValue = value;
});
console.log(firstValue); // 1

// you can just do this:
console.log(await singleton); // 1

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