niti: add write_attribute as a unique entry point

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: clean the code and add some comments

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: use perfect hashing for atribute access

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