For superclasses which have a non-invariant position, keep their position in attribute table

Property definitions

nitc :: virtual_machine $ MClass :: _positions_attributes
	# For superclasses which have a non-invariant position, keep their position in attribute table
	var positions_attributes: HashMap[MClass, Int] = new HashMap[MClass, Int]
src/vm/virtual_machine.nit:422,2--423,74