Implement LegacyStatementDeserializer
Normalize claim serialization on Items and use LegacyStatementDeserializer

Problem is that Items might have Claims (e.g. missing 'rank' and 'refs' keys).
Deserializing these now fails on Items. The serialization needs to be 'normalized'
as a Statement to deal with this potential inconsitency in old data.

@todo more tests and clean up of LegacyClaimDeserializer

Bug: 70613
Merge pull request #33 from wmde/statements

Simple fix for Item deserialization for items with claims