Reorganized light and surface traits so that light sources are
surfaces as well, which will let them slide easily into
intersection tests with the rest of the scene geometry.
Also created a proper World struct in the process, to store all
infinite-extent type stuff.
Note that I goofed and did a new rustfmt pass but forgot to
commit before making these changes, so there's a lot of
formatting changes in this too. *sigh*
