[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.~~
It’s worse than I understood actually:

> In particular, here’s the worst-case scenario:
> 
> Someone does this:
> 
> dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0) {
> 	modify(a) { /* some code that is normally very fast, but does a preferences lookup */ }
> }
> 
> eventually, it’s going to happen that that misses the preferences caches at the same time as a high priority thread tries to modify a. The high priority thread will spin on a, starving cfprefsd. At this point the entire device will hang.