Start work on prototypes

The discussion about possible variants is still active, but I still canalready start with implementing some prototypes.

The first approach will probably be to use dynamic fields for predicates to store the objects. This should allow for various features, but will only be practical for rather homogenous data, with (comparably) few different predicates. Therefore, this approach could be well-suited for defined drupal data, e.g. users or the like.

Individual prototypes will be introduced in different issues and linked here, and probably be programmed in different branches.