nitvm: Fixing a bad allocation of internal structures for attributes

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: Keep the positions of methods and attributes blocks in each class

Signed-off-by: Julien Pagès <julien.projet@gmail.com>

Conflicts:
	src/vm.nit
nitvm: Introduce a custom algorithm for ordering superclasses

Signed-off-by: Julien Pagès <julien.projet@gmail.com>