Clean up EntityDiff and ItemDiff

[JeroenDeDauw]

Moved here from wmde/WikibaseDataModel#209

EntityDiff makes assumptions that all entities have aliases, labels, descriptions and claims. It is generally not well designed. claim is used as key for statements.

Creating a well designed alternative from ground up might be the way to go.

[thiemowmde]

#92 and #61 do not solve this (however a final solution would look like) but are working on exactly this issue.