Add Statement::newFromMainSnak

Split from https://github.com/wmde/WikibaseDataModel/pull/205
So we will also add `newFromMainSnakAndQualifiers` as well as `newFromMainSnakQualifiersAndReferences`? That's what optional parameters are for. That's what constructors are for. Introducing more of these static method(s) means we will replace all `new Statement( ... )` calls with static `Statement::new...( ... )`. I don't see how this makes the overall situation better.