[WIP] Value objects

This pull request takes a different approach than GH-265 to implement ValueObjects. Instead of changing most of the code in every layer, we just inline embedded object class metadata into an entities metadata and then use a reflection proxy that looks like "ReflectionProperty" to do the rewiring.

The idea is inspired from Symfony Forms 'property_path' option, where you can write and read values to different parts of an object graph.

This is a WIP, there have been no further tests made about the consequences of this approach. The implementation is up for discussion.