The a* algorithm is more or less working! There are still fairly frequent
segfaults that I'm looking into, and the enemies aren't actually able to
act on the path yet, but you can see what tiles they looked at to arrive
at a particular path, which is pretty neat.

The reason enemies don't actually move is because I need to produce a linked
list of tiles that point from the end back to the beginning, but the scoping
of things is making a decent linked list pretty difficult.  But there you go.