Add new subscribe/drive/emit with object, Deprecate withUnretained on Driver

[freak4pc]

tl;dr

This PR will:

1. Hard deprecate withUnretained for Driver specifically
2. Update documentation for the other implementations of withUnretained so it’s obvious to look out for replaying behavior
3. Add new subscribe(with:onNext:etc:) operators for all items: Observable, Driver, Signal, Infallible, Completable, Single, Maybe.

Why are we doing this?

RxSwift 6.0 added a new withUnretained operator that creates a weak reference and then retains it within the next emission, so you could do observable.withUnretained(self) and get self with the element in parity.

Unfortunately there is a small side-effect we didn't consider. If the stream you're working with has share(replay: 1), the replay buffer would buffer self along with the value, which might create a retain cycle in some scenarios.

This in itself isn't an issue (just a documentation change), but for Driver specifically, the default is to replay the latest value, so we are forced to immediately deprecated withUnretained on Driver, specifically. It is still available on all other traits, Observable and Signal.

This is a very rare decision, to deprecate something in a patch release, but given this is a very new API, we prefer to deprecate it ASAP, before any widespread adoption.

Along with this deprecate, we're adding a new set of APIs to have a similar action for different pieces. For Driver, this looks like this:

driver
  .drive(with: self,
         onNext: { object, value in ... 
           // object is self, value is the emission from `driver`
         })

This API is more concise and is available for all pieces in RxSwift and RxCocoa: Observable, Infallible, Driver, Signal, Completable, Single, and Maybe.