A way to interrupt current process and save the best current reduction to a file

[pacak]

Explanation: it's been running for 30 minutes now, making good progress, but since I'm reducing a compiler memory leak and criteria - compiler reaching certain memory usage - it gets slower - I started with a big file so I had to set high cutoff point. Smaller it gets - smaller is the normal usage - longer I have to wait for a leak. With ability to restart - I'd set memory usage limit much lower at this point.

[TimNN]

To me it would also be interesting to occasionally see the intermediate state, to better understand how far it has gotten.

Both of these could maybe be solved with a --snapshot-file flag that, if specified, would write the current best candidate to a snapshot file every X time.