Add random binary tree generator to SwiftCheck

[lauraskelton]

• Generate random binary trees and validate/shrink them in SwiftCheck
• Use this to test DataStructures/Algorithms solutions
• Demonstrates power of generics in the library

[lauraskelton]

• Worked on rewriting binary tree in Swift
• Wrote a Stack data structure in swift to assist with tail recursion in the binary tree
• Wrote a tail recursive summation function for the binary tree
• Wrote a tail recursive print function for the binary tree

[lauraskelton]

• Added SwiftCheck extensions for ints, arrays of ints, arrays of strings
• Learned that the LazySubstring generator could be written in a more clever way using a struct or a class with a subscript syntax that could return a calculated substring for a given index in the not-yet-created array, using syntax such as arrayOfAllPossibleSubstrings[12] to give the unique substring that would be at index 12