[plt-scheme] two questions about extensions and 3m under 372

Thanks to Chongkai Zhu and Eric Kidd for their very helpful responses.

I think I'm going to try using "mzc --xform", along with some manual 
inspection.  If that goes smoothly enough and seems to work, then I'll 
stress-test the extensions to be reasonably confident they're not 
breaking the GC.