Replace some Claims with StatementList

I wanted to replace some usages of the Claims class but
immediatelly lost interest when I realized I would have to add
dozens, if not hundrets of these pointless wrappers:
new Statement( new Claim( ... ) ... )

This starts horrifying me. Code that has to use these classes
gets more and more messy. This is in no way better than simply
continuing to use Claim and Claims.

See https://github.com/wmde/WikibaseDataModel/pull/268

Similar problem with
new StatementList( array( ... ) )
It should accept a single statement.

Change-Id: Ia9ce8770e05da1477c082c9f345ffe596489f564