Method get_stack_const

Increasingly, Cory suspects that this is the wrong way to do this... Instead we should: 1. Check if the expression is a constant. Reject as a stack delta. 2. Check if the expression is a simple variable. Accept as 0 if esp_0, else reject. 3. Substitute esp_0 with the value zero. 4. If the expression is different, it's the "stack delta".