[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.~~
> There should only be one `Disposable` per `Consumer` here.

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?

> I don't know what this means:
> 
> > and without making it the callers’ problem.

I meant that one virtue of the code you’ve written is that I as a client of the API don’t have to care about its allocation patterns; that’s a virtue worth preserving :+1: