Simple fix for Item deserialization for items with claims

it's apparently possible to have Items with Claims and deserialization of this is broken.

This fixes the issue with a more minimal, bug fix solution.

A LegacyStatementDeserializer is added and used by the LegacyItemDeserializer.  The LegacyClaimDeserializer is not touched and still does the same thing, so not a breaking change.

See https://github.com/wmde/WikibaseInternalSerialization/pull/32 for follow up (or alternative) to this pull request.