Move the ENTROPY sig to evented semantics

As per discussion with @avsm .

Since the idea of `ENTROPY` is not to provide random numbers, but environmental noise, and since this noise is opportunistic and not predictably available, instead of giving entropy on demand, the device now emits entropy when it has some.

Goes with the [corresponding](https://github.com/mirage/mirage-entropy/tree/event-driven) change to entropy providers.
Oh, but that's what I meant by "no other meaning" -- source is really just used for mixing. It has zero meaning here, apart from being an integer.

[This](https://github.com/mirage/mirage-entropy/blob/event-driven/unix/entropy_unix.ml#L67) is basically its semantics.