Fix the build issues when using the Xcode 9 GM.

[ehyche]

This change fixes this issue: #51

The build issues include:

• KeyPath class was added in Swift 4, which collides with the KeyPath defined in JSONUtilies.
  This was fixed by disambiguating the type by specifying "JSONUtilities.KeyPath" instead of just "KeyPath".
• DecodingError was added in Swift 4, which collides with DecodingError defined in JSONUtilities.
  This was fixed by disambiguating the type by specifying "JSONUtilities.DecodingError" instead of just "DecodingError".
• There were several places where xcodeproj was expecint [String] and the code was calling it with Set. Apparently this worked in Swift 3.2, but no longer works in Swift 4. So these places were corrected by:
  • Not creating a Set() when an array was needed
  • Calling .sorted() on the Set to produce an array

[yonaskolb]

Thanks for the PR @ehyche! Unfortunately you're about an hour late! 😄
The Swift 4 #52 has already been merged.

In terms of the xcodeproj changes, that's because you must have linked to a new version as Swift 4 doesn't read Package.pins anymore so it upgraded itself. I noticed that as well and actually locked it to the previous version in that PR. I'm now going to do the xcodeproj update

[ehyche]

@yonaskolb : Wow - I looked at the open PRs before I started mine, but I guess you had already merged by then!

By the way, I also noticed that Spectre also did not build on Swift4, so I submitted a PR against that project to fix it: kylef/Spectre#29. However, it looks like there was already a PR open since June to fix the same issues. So I'm not sure if that project is still being maintained or not.

[yonaskolb]

Spectre builds ok for me. There are some warnings in Swift 4 which is what I think you're referring to, but that's not a breaking issue for this repo 👍

[yonaskolb]

The upgrade to xcodeproj 0.1.2 has also now been merged in #56