[swift2objc] Support Extensions

[nikeokoronkwo]

Extensions are a swift language feature that allows you to add methods and other nested declarations to a swift nestable declaration (struct or class). For exporting to Objective C, only classes are allowed to be extended (denoted by @interface <class-name> (SWIFT_EXTENSION(<module-name>)) in Objective C)

extension A {
  public func newFunc(p: String) {
    // function body
  }
}

They work similar to the relationship between Dart classes

Swift functionality present in an extension usually have a swiftExtension field in the generated symbol graph

[liamappelbe]

This feature sounds like ObjC's categories.