Event driven

The other side of [evented entropy](https://github.com/mirage/mirage/pull/261).

The interface now feeds entropy to a provided handler.

On the Unix side, this means spinning up a timer-wheel and simply reading `/dev/urandom` periodically. Which should instead be done after other events fire, to avoid needless wakeups. The handler is fed some initial entropy as soon as it is installed.

On the Xen side, we still just use built-in (soft) RNG and not even bother spinning the timers. This provider should start handing out time measurements and such from drivers, later on.
It's intended to disappear as soon as we can hook into the Lwt_unix select loop -- we can just reseed the pool every time we wake up from select with a quick read of `urandom`