Allow <nameConstraint> and <valueConstraint> without constraint= attribute.
This is a valid usage if the value must be of speficic type, but no
additional restrictions are needed or supported by the type checker.
Add set/get_name_constraint() methods for tag nodes.
Remove the argument set_tag/set_leaf/set_multi methods,
make them simply set the property to True.
Fix value handling in set_leaf/tag/multi methods.
Add get_value_constraints() method, fix property name.
Add unit tests for ReferenceNode.