OCaml 4.12 and chibi

Not making a ton of progress on this port, but the changelog talks a lot about changes in the memory model, so it's probably worth resetting to the new version.

On the chibi front, I abandoned the original mkfile (for better or worse) and added a generator rc script. mk doesn't like running subshells in certain instances (that I'm not clear on), but splitting out the generation into a script avoids all that weirdness.

I actually feel pretty close to a workable solution as far as the chibi rework goes, but we'll see what new issues crop up.