Add fuzz-testing

[skx]

Looking at the code I'm embarassed by easy crashes. For example I had to make a fix to get #98 to even try running - by default you get a panic.

Add basic fuzz-testing, and keep running it until we get 1 minute of crash-free operation at least.

[skx]

Closed with an initial implementation that has run, happily, for 5 minutes.

No doubt there are more crashers out there, but this will do as a first-pass.