code: public static <T> Key<T> of(Constructor ctor)
    {
      return new Key(ctor.getDeclaringClass(),
                     qualifiers(ctor.getAnnotations()));
    }
docs: Builds Key from constructor's declaring class and constructor's qualifying
  annotations. A qualifying annotation is marked with a {@code @Qualifier}.
  
  @param ctor constructor for deriving type and annotations from
  @param <T>  target type
  @return instance of a Key