Vats: repl commands for starting, stopping, inspecting

Fibers has some nice comma-commands for starting/stopping fibers/schedulers/etc. We should follow their lead. Whenever a vat is created, it should appear in some sort of set that can be explored at the REPL.

Idealized feature set (strikethrough means they are done now):

• List all active vats
• Check a vat's queued waiting messages
• Stop a hung turn/churn
• Turn on/off certain debugging features
• Execute code against a particular vat
• Start logging/capturing incoming messages
• Start logging/capturing uncaught exceptions
• Time-travel debugging against a previously uncaught exception

Probably many of these should be broken out into sub-issues.