Discussion: Support "Subscribable"?

[benlesh]

Mostly putting this here as a flag in the ground because it comes up.

Internally, we need to subscribe to "Subscribables". That is, essentially, the return value of [Symbol.observable](). Or the following interface:

interface Subscribable<T> {
  subscribe(observer: Observer<T>): { unsubscribe(): void }
}

As it stands, we don't support this as a return type from any rxjs function. If you had a type like this, you'd need to decorate it:

mySubscribable[Symbol.observable] = function() { return this; }

Questions:

1. Do we want to support this with from, mergeMap, etc?
2. How does our future with regards to AbortSignal et al, effect this?