unique identifier for any object

 Object::ID is a unique identifier for any object, regardless of its type,
 structure or contents. Its features are:
 .
   * Works on ANY object of any type
   * Does not modify the object in any way
   * Does not change with the object's contents
   * Is O(1) to calculate (ie. doesn't matter how big the object is)
   * The id is unique for the life of the process
   * The id is always a true value