nitvm: Introduces a type which represents initial values inside attributes

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: Throw an error when trying to read an uninitialized attribute

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
nitvm: Handles isset expression

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