Now :entities of the map hash-table is no longer a
flat list of entity plists, but a “chunked” alist
of these entity plists. Each alist is dedicated to
a different “chunk” (screenfull) of the map.
This (hypothetically) makes finding on-screen
entities more speedy.
No functional changes.
Re-order functions in overworld.tiled so SBCL
doesn't complain at me.
Split some functions from overworld.tiled into
a new overworld.util package. Also rename the
files in a nice way!