Add better API for Disposable when adding to DisposeBag

[andyyhope]

Added

I brought up in the Slack channel that the original Disposable API .addDisposableTo(_ bag: DisposeBag) felt out of line with the newly minted Swift API guidelines.

Because an Observable returns a Disposable after using the subscribe API, it makes no sense to treat the Disposable API like it's an Observable.

After some discussion in the channel, the group generally favored disposed(by bag: DisposeBag) to eventually replace addDisposableTo, which is why we've created it as an extension for the time being. People will be able to slowly transition from the old API to the new without being under duress.

The new APIs implementation is merely a wrapper around the old. I wrote it this way because I felt keeping one implementation of it would be beneficial, at least until the old one gets deprecated (if it ever will).

Old API

.addToDisposeBag(disposeBag)

New APU

.disposed(by: disposeBag)

[RxPullRequestBot]

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