Remove static `create` methods and provide importable functions instead

[benlesh]

The static create methods were a good idea, but they suffer from a couple of issues:

1. TypeScript has a hard time typing them because of how static inheritance works. For example Subject inherits from Observable and BehaviorSubject inherits from Subject... all of them have (or should have) wildly different signatures for create.
2. Because of how inheritance works, poorly implemented subclasses (like BehaviorSubject suffer from bugs like BehaviorSubject created with "create" does not have getValue() method #1890.
3. Subject.create doesn't even do what the rest of the create methods do (which is just proxy the constructor), instead it returns an AnonymousSubject, which is really just any old observer "glued" to any old observable to make a subject. So we might want to change that to createSubject or frankenSubject or glueSubject (okay, I'm kidding on the last two) (but the name AnonymousSubject sorta sucks too)

Proposed solution

Create functions that do the same work and export them separately:

import { observable, behaviorSubject, /* et al */ } from 'rxjs';

// usage would be

observable(subscriber => {
  subscriber.next('hello world');
  subscriber.complete();
});

[kwonoj]

if we're going to away from exposing creation method directly, what makes us prevent just let ppl use class ctor instead of exporting another function for creation?

[benlesh]

I think this is a non-issue these days. Closing for now.