[swift2objc] Support tuples

[liamappelbe]

Like Dart records, Swift tuples can have positional and named elements. The main difference is that to access the first element of a Dart record you write foo.$1, whereas in Swift you write foo.0.

Tuples are easy enough to parse, but they're not supported in @objc annotated APIs. So we'll need to transform them into classes.

(Int, String, abc: MyClass)

// Would be transformed to something like:

public class Tuple_Int_String_abc_MyClass {
    public let _0: Int
    public let _1: String
    public let abc: MyClassWrapper
}