Implement the pickler in C++ for better performance

* More than 60 times faster than the Oz pickler!
* Made available to Oz as the builtin Serializer.pickle.
* Introduces a few core atoms to ensure fast comparison.
* Uses an algorithm close to the one of Serializer.
* Handles resources and read-only variables as the original pickler.
Use the new serializer for inter-VM operations

* Also removes dependency on Pickle so the whole VM module is serializable!
* No need anymore for the 'pickle.pack' property.
* Sending unit on a VMPort is 50 times faster.