[WIP] The Great Swiftening

**: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.~~
Would it yield a more usable API if we were to curry the evidence? I tried sketching out an implementation of this but I was having a little trouble thinking out how it would look, so starting from that end:

```swift
producerOfProducers^.merge()
```

Here I’m hand-:wave:ingly assuming that postfix `^` lifts `Producer<Producer<T>>` to a `MetaProducer<Producer<T>>` wrapping its argument, which implements `merge()` &c. directly (since it knows it’s always higher order).

A bit like the [higher-order destructuring](https://github.com/robrix/Hammer.swift/blob/master/Hammer/Destructurable.swift) trick I employed in Hammer for destructuring one level of recursion deep.