I don't think this is needed right now, but it comes up a few times in Olli's CC patches. It would be nice to have a little function that sees if a jsval is a gray GC thing, so people don't have to worry about the nuances of .isMarkable vs .isGCthing, and so we sweep up all the weird possible cases.

I don't remember where exactly this was going to happen, but I guess the place this was being used got removed somehow or another.