[WIP] The Great Swiftening (a.k.a. the new 3.0)

**:warning: NOT READY TO MERGE :warning:**

Resolves #1365 and #1366. Per my comments in #1369, this is intended to be a long-running branch that will eventually become RAC 3.0, supplanting the [current `3.0-development` work](https://github.com/ReactiveCocoa/ReactiveCocoa/pull/966).

To get us started, I basically just subtree merged [jspahrsummers/RxSwift](https://github.com/jspahrsummers/RxSwift) into here, then bridged several RAC classes back and forth.

~~Currently, only the Mac framework target builds successfully. We may need to revamp our build settings in order to combine Swift and Objective-C correctly.~~
> Does that mean that if I make a long signal chain where the intermediate steps aren’t used elsewhere, I’m only going to encounter one `Disposable` for the whole chain?

Ah, no, there will be multiple. I'm intent on making disposables and consumers lighter-weight than they are in Objective-C, though, through gratuitous use of `struct`s where possible. There's still some work to do on that front, but I think the situation should already be improved over what RAC 2.x offers.

I don't necessarily agree or disagree with the reducers idea, so I'd be interested to see a proof-of-concept within this framework. In any case, I don't think there's anything to prevent us from adding it later.