static Optional<ExecutableElement> toPrettyStringMethod(
      TypeElement element, Types types, Elements elements) {
    return toPrettyStringMethods(element, types, elements).stream().collect(toOptional());
  }
Returns the {@link com.google.auto.value.extension.toprettystring.ToPrettyString} annotated
method for a type.
value/src/main/java/com/google/auto/value/extension/toprettystring/processor/ToPrettyStringMethods.java