Added a struct for returning the length of the alphabet as well as the
alphabet when creating an alphabet from an interval
Made stack contain an array of structs and added a free_stack() method
Updated stack usage to new model and added free_stack() to end of
iterate()
Minor bugfix in push()
Removed inline from free_stack()
Reintroduced target to create_substring()
Minor bugfix in create_c_array()
Fixed tests