Tombstone is not a primitive value

Mark I know this isn't quite what you had in mind, and I'm not at all against cleaning up the type hierarchy. But first I fear that I might have made a mistake with the position of tombstones in relation to struct/list (and worse the thrift formats).

Correct me if I'm wrong/smoking-crack but does it make sense for struct/list values to be tombstones? I don't think it does.

If I'm not crazy then I might need to tweak the ingest thrift file a little more.