Reworked initial design

• instead of "obligatory" and "selectable" fields, for simpler cases all fields can be wrapped in the same F[_] - where and set part will be done using different objects in update query instead of one with different roles of particular fields,
• this way for simple vases you need only one F[_] - another one is needed only when you your create object differs from fetched entity because some fields are generated by database,
• fetching supports now pagination,
• queries in Repo are now generated by utilities that can be used by API user as well.