Refactor the type system

[vinivendra]

There are several complicated algorithms (many of which haven't been implemented yet, like #63) that would be easier to do if we had a decent type system.

Instead of using strings for all types, we could have a more complicated structure. Something like

indirect enum SwiftType {
    case simpleType(String) // `Int`
    case functionType(parameter: [String], return: String) // `(Int) -> Int
    case tupleType([String]) // (Int, Int)
    case dotType(left: String, right: String) // String.Index
    case genericType(type: String, genericParameters: [String], constraints: [TypeConstraint]) // Array<Element> where Element :Equatable
}