FWIW, asdl_c.py could use some "modernization", but I'll defer this to a later cleanup in order to do things gradually.

The same can be said for the Makefile rules - they can be simpler and more efficient (no need to invoke asdl_c / parse the ASDL twice, for example).

Incidentally, where would be a good place to put the ASDL tests? Can/should we reach into the Parser/ directory when running the Python regression tests (will this even work in an installed Python)? Or should I just plop asdl_test.py alongside asdl.py and mention that it should be run when asdl.py changes?