Basic implementation of references between documents.
* Refactoring or collection part implementations.
* Some cleanup of usage of part builders for better part creation.
* Refactoring or compound part implementation.
* Some cleanup of usage of part builders for part creation.
Some cleanup of usage of part builders for part creation.
Removed dependencies of storage implementation from core.
Some cleanup of usage of part builders for part creation.
Implementation of equals and hashCode methods for document parts.
Rework of id handling.