drive() and emit() for multiple observers

[freak4pc]

To align with bind(to: Observer...) (#1702) this PR adds the same abilities to drive() and emit().

I also cleaned up some of the implementation (removed an unneeded private method) and removed some duplicate tests for Driver.

[kzaher]

Hi @freak4pc ,

wouldn't this change our ABI?

[freak4pc]

Hey @kzaher,
The code is backward compatible (e.g. drive(observer) works alongside driver(observer1, observer2))

The method signature itself is different which might mean it's not binary stable - but I'm not sure that's an entirely huge issue as long as we retain the same execution context.

If it's a big deal we can easily leave the old signature and relay it into the variadic one.

[freak4pc]

@kzaher I think this can be a good time to add this.

[pluddy]

This would clean up quite a bit of code repetition we have going on as well. Any way we can keep the conversation going on this @kzaher @freak4pc ?

[freak4pc]

This would be a good addition to RxSwift 6 which should be quite soon. ABI breakage shouldn’t be an issue in that case.

[juanjo-ramos]

I just saw this c3c0cac.

Does it mean this PR won't be included in RxSwift 6?

[freak4pc]

No:

1. It’s just a beta
2. It wasn’t released yet