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