[Swift 4.1?] Add Arbitrary instances for EVERYTHING

[CodaFi]

With the merger of SE-0075, we should be able to conditionally export new Arbitrary API for every module under the sun. Off the top of my head

• Foundation
• UIKit
• AppKit
• CoreData

would be useful to start with.

[bgerstle]

I've already got some generators for CoreGraphics & UIKit that I'd be happy to contribute.

[kykim]

I've got some Foundation (pre-Swift 3) generators that need to be cleaned up, but I'd be happy to contribute, as well.

[adamkuipers]

Do you think these would be better separated into different projects? It would keep the API footprint and code size small for SwiftCheck proper.

[CodaFi]

Now that it's been merged for real this time @kykim do you still have those Foundation extensions?

[bgerstle]

I'll try to put together something w/ my CG & UIKit generators as well.

[CodaFi]

Fantastic! It's going to be a bit awkward to work with an old toolchain on this stuff, but I have a feeling this is going to ship with 3.1 in January-ish so it shouldn't be that long!