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.
.... and [here](https://github.com/mirleft/ocaml-nocrypto/blob/master/src/fortuna.ml#L105) how it's used. The idea is to exploit indeterminate ordering of provider's firing for a little more unpredictability.