nitvm: Migrating some init to new-style constructors and fixe a missing doc warning

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: Method dispatch is implemented with direct access

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: Subtyping test is now implemented with direct access

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