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