Swift 3 Trie

[kelvinlauKL]

Also noticed that there are a lot of inconsistencies between the RW Swift style guide and the current implementation.

Going to refactor the Swift file in the playground. Here are a few changes that I hope to achieve:

• Usage of computed properties rather than methods as much as possible. The current implementation still relies on Objective-C styling, which all getters/setters are required to be methods.

Here's an example of such a change:

// old implementation
func isRoot() -> Bool {
  return character == ""
}

// new implementation
var isRoot: Bool {
  return character == ""
}

• Updating the naming for functions / parameters.
• Updating method signatures - Will need a bit more investigation, but I feel the reliance on tuples is unnatural in this scenario.

Many of the methods return a tuple. For example:

func isPrefix(_ prefix: String) -> (node: Node?, found: Bool) { ... }

isPrefix is a method that checks the Trie to see if the argument exists in the Trie. If it exists, the method returns a node object and true. If it doesn't exist, the return is nil and false. The value the optional makes the boolean unnecessary.

• Various little styling changes.
• Creating a playground for the Trie implementation.
• Updating the readme appropriately.