Fix calling required extractors through traits

[EliasC]

The following program generated faulty C-code:

trait Result<b> {
  require Success() : Maybe b
}
passive class Success<b> : Result<b> {
  data : b
  def init(data : b) : void {
    this.data = data
  }
  def Success() : Maybe b {
    Just this.data
  }
}
class Main {
  foo : Result<int>
  def main() : void {
    this.foo = new Success<int>(5);
    match this.foo with
      Success(x) => print("It's a success! The resulting value is {}\n", x)
  }
}

The problem was that the extractor code was assuming that the extractor
was a method defined in a class, so it would not work for trait types.