Uses setitimer() to set a recurring signal, and prints
a Python stacktrace at the next safepoint aka allowGLReadPreemption.
This is not great since allowGLReadPreemption can happen a decent
amount later than the signal.  (I'll play around with trying to get
the signal to be acted on sooner, but it might be better to wait
for full signal-handling support.)

Still, it seems to provide some decent high-level info.  For example,
half of the startup time of the django-template benchmark seems to be
due to regular expressions.