Heaps of changes to osc

I've been working on osc quite a bit lately, and some new features (and, of course, bugs, which I'll work on soon) have been added.
There's arrow key support (finally!), checkpoints, mildly challenging enemy AI (if you can call it that), support for loading levels which have been designed in the level editor with l + any number key from 1-5, and sounds.
I've made a video showing some of the new stuff.