Improve logging

[xaprb]

Robustly doesn't give much insight into what happens inside it. We should probably accept a logger, or use Go's log package. As an alternative, we could instead define a package variable that's a channel, on which messages can be sent in case someone wants to listen to them.

The types of events to capture:

• Starting to run a function
• Function exits nicely
• Function panics, and the stack trace, rate and duration
• Function is restarted
• Function panics too often, and we re-panic