Only compress the selected modules and die gracefully on Ctrl-C

The only downside remaining is a bit of console spew after you Ctrl-C a running scriptb because it orphans its children. I don't think that's a serious issue.

Output from subprocesses is also potentially interleaved, but it's rare because completion time has high variance and only 4 run concurrently. I don't think anybody is reading the output, anyway.

On my MBP, parallelization increases CPU utilization from 60% to 100%, but it doesn't significantly impact runtime (2 minutes ± 10 seconds either way). I'd expect that the speedup would be more significant on a larger host. There's a 55% increase in CPU time consumed in the parallel version over serial, so it seems that the parallelization is basically a wash on my laptop.