Request: Define constants for order states in commerce_order.module

I was looking at code and found that the commerce_payment module defines constants like COMMERCE_PAYMENT_STATUS_PENDING. There are no such constants used for commerce_order module defined depsite the frequent use of 'canceled', 'pending' and 'completed'

I would suggest creating

COMMERCE_ORDER_STATUS_PENDING='pending'

COMMERCE_ORDER_STATUS_COMPLETED='completed'

and

COMMERCE_ORDER_STATUS_CANCELED='canceled'

so that others building up modules that hook into the commerce_order.module aren't hard coding, can catch typos more easily, best-practices, etc.