v1.5.0

serialization performance (#79)

Significantly improve serialization performance
* Added serialization benchmark test
* Inline some methods that are in the main loop
* Switch from linq to for loops
* Do not add resource objects to the Included collection
* A slightly better GetHashCode for ResourceObjectReference
* Introduced ResourceObjectContract to determine type metadata prior to serialization
* Determine relationships by keep a stack of resource objects rather than using writer path