COMPARE FUNCTIONS

Q: Where do the compare functions and delete functions come from?
A: Whoever calls your code must provide those functions. 
   - Since your test_priority_queue. will call your code, then it you must provide
     compare/delete functions in your test_priority_queue.c.
     WARNING:  compare/delete functions go in your test file, not in priority_queue.c.

Destroy function frees whatever the node refers to, only if needed.
- If you have a linked list of strings on the heap and you want the strings to be freed
  whenever you destroy the list, you would pass a destroy function that just calls
  free(node -> a_value) or something like that.  If you don't need the stuff to be freed,
  you pass an empty destroy function.