I'm a bit annoyed to need maybeCompartment, since we should be able to statically determine if there is a compartment method on T. Unfortunately the tricks I used to capture this were universally longer or more complicated than this approach, or crashed gcc, clang, or gdb. On the other hand, this approach does let us effectively remove the MaybeCompartment method in jsgc.cpp. That patch is a bit lower in my queue, but does make this approach end up paying for itself pretty handily.