code: public static <T> Builder<T> newSorter(Serializer<T> serializer, Comparator<T> comparator) {
          return new Builder<T>(serializer, comparator);
      }
docs: Fluent API building a new instance of ExternalMergeSort<T.
  @param serializer Serializer<T> to use when sorting.
  @return Config instance that can be used to set options and in the end create a new instance.