Make Optionalish available for use by extensions.

Is there a reason [Optionalish](https://github.com/google/auto/blob/1d316fa504f96a9caa5ac93a6074c759cd6140f3/value/src/main/java/com/google/auto/value/processor/Optionalish.java) shouldn't be visible outside of the com.google.auto.value.processor package? If extensions shouldn't use those classes directly, maybe Optionalish can be moved to common. It would certainly be useful to work with optionalish types from extensions without making a copy.