Adding operate

[benlesh]

This is the execution of the design from #7253.

Please review per commit.

The work here is broken into commits to help understand the changes.

The key bits of work are:

1. new Subscriber now accepts an overrides argument as the second parameter. This argument contains the same set of arguments we used to pass to createOperatorSubscriber. In short, passing in next will "override" _next, error overrides _error, etc.
2. Subscriber constructor has been re-deprecated (it's currently deprecated in 7.x). There's no reason anyone should be creating a subscriber instance with new Subscriber, really.
3. The new Subscriber constructer with 2 arguments is marked @internal, and is not a public API.
4. A publicly exposed function called operate has been added. This basically wraps the new Subscriber constructor instance.
5. Operators were updated, one-by-one, in separate commits. There are no functional changes to these!!, all that was done was moving from createOperatorSubscriber(destination, next, complete, error, finalize) to operate({ destination, next, error, complete, finalize }). That's it.
6. Only one test needed to be updated, and it was testing the createOperatorSubscriber as a means of creating operator-related Subscriber instances and subscribing to a Subject with them.

If it helps reviewers, I live-streamed the bulk of the work here: https://youtu.be/YHyt3fHIEAk

• Updating Subscriber to accept the overrides work and explanation: https://youtu.be/YHyt3fHIEAk?t=475
• Adding operate function: https://youtu.be/YHyt3fHIEAk?t=1304
• Updating Subscriber signature type overloads: https://youtu.be/YHyt3fHIEAk?t=1491
• Adding unit tests for operate: https://youtu.be/YHyt3fHIEAk?t=1718
• Deprecating and internalizing Subscriber signatures (with dtslint testing): https://youtu.be/YHyt3fHIEAk?t=3420
• Example: updating catchError: https://youtu.be/YHyt3fHIEAk?t=4210
• Example: updating takeLast: https://youtu.be/YHyt3fHIEAk?t=4454
• This is where I realized I needed to make sure the finalize override/handler is fired after the subscriber cleans up: https://youtu.be/YHyt3fHIEAk?t=6368