SortedVIntList should be Serializable [LUCENE-2697]

For some memory intensive searchers we have, it is important to temporarily store the created bitsets to disk
SortedVIntList does not implement Serializable: since it could be easily serialized due to the raw data it contains, it should implement it

We are currently working with a custom SerializableSortedVIntList: we've just copied the code and added Serializable but we would definitely like to stick on lucene official code rather than such customizations

Hope you will accept the proposal

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Migrated from [LUCENE-2697](https://issues.apache.org/jira/browse/LUCENE-2697) by Federico Fissore, 1 vote, updated Aug 10 2011

Start by locating the SortedVIntList class and reading how its raw data is stored and exposed. Check nearby tests or serialization patterns in the project, then make the class serializable so instances can be stored and restored for disk-backed use; done means the official class supports that round trip.

Written by the indexing model from the issue text.