Fixed the Makefile to work with the new source code organization and
also added back one include that was commented out erroneously. Next
commit will get rid of diff.c/h.

-Peter Mawhorter
Removed the diff code as I won't be using it.

-Peter Mawhorter
Added a queue datatype (which seems to have a memory corruption bug
atm!). Also split out exposure computation into its own file. Still
working on wrangling loading so that chunks are only compiled when their
whole neighborhood is loaded, and thinking of moving to a different
model for storing loaded chunks.

-Peter Mawhorter
Made use of "const" clearer throughout the code by putting it after the
type it modifies and made many pointers const in addition to their
targets. Defined a new unit testing framework and wrote some tests for
lists and queues, resulting in some bugfixes (especially once valgrind
got involved). In particular fixed a memory corruption bug with
grow_if_necessary in datatypes/queue.c and a memory leak due to a
missing free in test_compile_frame.

-Peter Mawhorter