(1) Introduce tracegen.py, a script that invokes the emulator (built
with TraceGenConfig), sending a SIGTERM once all cores are finished.
(2) Update toaxe.py to gather some statistics about the trace.
(3) Introduce tracestats.py, which displays the stats in a useful way.
(4) Introduce tracegen+check.py, a top-level script that generates
traces, checks them, and emits stats. If this commit is pulled, it
should be done after pulling my latest groundtest commit.
