boarding_queue.c: Created and added; this resulted from me thinking about how to model queues to board airplanes or get through security.  It's honestly a slightly dull example, but I could see it being useful at some point.  As it is, it uses an idea of delays over an idea of priority; the higher a delay, the more need to satisfy it. (Currently that requirement only needs to be satisfied for one element, but I can foresee replacing the current method with a function-based approach.)
boarding_queue.c: Removed the whole 'unsigned' business for now; decided that had no place in the current form of the system.  After all, keys might be replaced with a different type.  Everything is currently pretty nice; I'm thinking it'll learn some new tricks after I can come up with tricks to add.  Maybe a load-balancing non-priority adding method.
boarding_queue.c: Added the rest of the function declarations.  Now I'm looking for ways to extend this.