[ORM] Optimise ModelQueryBuilder::with for nested relations

[woru]

User::where(...)->with('role->group')

could load user's roles and roles' groups with one query.
We can analyze nested relation in 'with' and if they are of type belongsTo or hasOne (collection=false) join them instead of issuing separate queries.

[woru]

I mean join roles and groups, not users, roles and groups