Print method for displaying instances on the screen

The public print() method of an instance is called, if present,
from print.R6, the generic R6 print method. This way classes can
define how they want their instances  printed.

This is for #5. If you think it is OK this way, then I can also add
this feature to the manual and the README.
As you wish. I thought that since portable classes are stored differently, it does not hurt to test things with both representations.