Migrate to Swift 4

[edjiang]

Migrate to Swift 4 using the auto migrator + manual audits of the migration.

The primary impact of Swift 4 on our codebase is that Swift 4 no longer automatically generates Objective-C entry points for subclasses of Objective C objects. Since our codebase is meant to be consumed from Objective-C clients as well as Swift only clients, we need to add @objc annotations to tell the compiler to still generate these entry points.

See:

• https://github.com/apple/swift-evolution/blob/master/proposals/0160-objc-inference.md
• https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160509/017308.html

Also, the private access control modifier gained a bit of scope, making fileprivate unnecessary for the most part.

See:

• https://swifting.io/blog/2017/05/30/43-bye-bye-fileprivate/
• https://github.com/apple/swift-evolution/blob/master/proposals/0169-improve-interaction-between-private-declarations-and-extensions.md