Better Swift support

[MaddTheSane]

As it stands, XADMaster uses a lot of exceptions, which Swift does not like (there’s no exception handling in Swift, and if an exception is caught by the Swift run-time, the program crashes).

Swift, instead, throws errors instead.

In order for Objective-C methods imported into Swift to throw errors, they must have an NSError** parameter and return either BOOL or a nullable pointer.

You can look at my fork for how I did it, but be warned: my code might meander all over the place. Also, a lot of my changes are Apple Platforms-specific, including generics metadata, nullability metadata, and Blocks.