[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.~~
Yep, just trying to think of a better name for it. `distinctUntilChanged` isn't horrible, but I think this (stolen from Elm) is better.

`uniq` is pretty bad, since it implies that only elements unique across the _whole_ stream are preserved, which isn't the case.