(nice-to-have) user code log rotation

[kierdavis]

Currently log.txt is rewritten every time the user code is run, obliterating any previous contents. It wouldn't be too difficult to instead write a new file on every run (e.g. logs/TIMESTAMP.log), perhaps also updating a symlink logs/latest.log to point at the most recent file. This makes it easy for a user to look back over the results of previous runs.

[RealOrangeOne]

Unfortunately, using a timestamp may be misleading, as the chances of the clock on the Pis being correct is rather low.

Also, symlinks aren't especially cross-platform, especially with Windows.

We might be able to write something which splits the log stream between 2 files, a log.txt and logs/4.txt.