Add smaller operator creation function.

[benlesh]

This work is blocked on finishing #5729 reviews and landing it... but the crux of it is simple enough... this pattern exists in all operators:

export function someOperator<A, B>(...args: any[]): OperatorFunction(A, B) {
  return (source: Observable<A>) => lift(source, function (this: Subscriber<B>, liftedSource: Observable<A>) { 
      // Do things here and maybe return teardown.
  });
}

The idea is to go through and replace the return here with a helper that does this wiring for us:

function operate<A, B>(init: (subscriber: Subscriber<B>, liftedSource: Observable<A>) => TeardownLogic) {
   return (source: Observable<A>) => wrappedLift(source, init);
}

usage:

export function someOperator<A, B>(...args: any[]): OperatorFunction(A, B) {
  return operate((subscriber, liftedSource) => { 
      // Do things here and maybe return teardown.
  });
}

Then use it in each operator to further reduce size. It may also help with readability? Either way, comments will be required.

[benlesh]

We may be able to remove the lift and wrappedLift implementations in this process via inlining what they do.

[benlesh]

I may also consider switching the order of the arguments to liftedSource, subscriber, because I feel like it makes more sense (although it's arbitrary)

[benlesh]

I'm trying to consider whether or not this helper function is generic enough to provide as a public API. My gut tells me that we should not do this, at least at first. However, it seems abstract enough to be mostly future proof, but perhaps not if we move to AbortSignal?

The signature does indeed work in an "unlifted" scenario (if we were to remove lift):

function operate<A, B>(init: (subscriber: Subscriber<B>, liftedSource: Observable<A>) => TeardownLogic) {
   return (source: Observable<A>) => new Observable(subscriber => init(subscriber, source));
}