Add a blocked task watchdog

In this stackoverflow question, someone gets very confused because they're using time.sleep and never using await, so one of their tasks seems to run correctly but the other doesn't. There's no indication of what's going on. Easy to fix once you know how, but if you don't, then how are you supposed to figure it out?

We could, pretty cheaply, add a watchdog to catch such situations. What I'm imagining is that when entering trio.run, we'd spawn a system task that spins up a thread and watches for the main loop to become stuck. In the main run loop, we'd send it messages occasionally, whenever we (a) start processing a batch of tasks, (b) stop processing a bunch of tasks. If the watchdog thread detects that the time between (a) and (b) exceeds, say, 5 seconds, then it should print an informative message and use the faulthandler module to dump everyone's stack traces. I think this might be cheap enough it could be enabled-by-default.

Start by tracing the code entered by trio.run and the main run loop, focusing on where task batches begin and end. Evaluate the proposed watchdog and faulthandler integration; done means a blocked loop exceeding the proposed threshold produces an informative message and stack traces without disrupting normal operation.