Stub timer op, for creating a timer.
Add a libuv-managed event loop thread.

This will be used in timers, async I/O, file system notifications, and
so forth. It will deliver completion notifications into a queue - one
that is presumably being used as a thread pool input stream.
Implement timers.

Doesn't do the required cleanup or have cancellation support yet, but
it's enough to pass an unfudged scheduler.t.