Create a typed event reporting mechanism

Tor provides "control port" mechanism for exposing generic events to an external application. It would be valuable to have similar function in Arti for reporting things like:

• changes in bootstrap status (see #96 (closed))
• guards becoming disabled
• security warnings and maybe some other stuff that the control port currently exposes.

This mechanism needs to be efficient, extensible, and easy to use. Possibilities include an asynchronous Stream of events, or maybe something kludged out of the tracing API.