nitvm: Clean the code and add an absolute offset for methods and attributes.

This offset will be used to implement object-mechanisms by direct access

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: Implementing attribute access with direct access

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: Attributes access is fully functionnal with direct access when possible

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