OK, I'm down to a single bug to be solved to call this work a "success" (there are other test failures, but they are not overtly difficult to solve).

At this point the bootstrapping is failing in the face of sub-interpreters. Specifically, when I try to load the frozen importlib code in Py_NewInterpreter() it leads to an assertion failure in the GC code when handling references in a GC generation. I have zero experience with sub-interpreters and my GC experience is rusty, so if anyone can have a look at the code I would appreciate it to see if they can figure out why loading a frozen module (while in marshal) is leading to a GC assertion error.