Make it possible to generate schedules

I understand that running `IOSim s` computations is purely deterministic: Every run will trigger the exact same scheduling of threads. This is great as it provides fast execution, but unless one wants to invest time and energy in `IOSimPOR` it does not actually help us spot the kind of problems in concurrent code stemming from "surprising" reordering of executions.

It seems to me that quick-checking code with `IOSim` would be even more powerful if the runtime was able to randomly select among runnable actions, generating and covering more possible execution schedules. I have only a vague understanding of io-sim's internals but this could be exposed at the API level through a function with an additional argument, like:

```
runSimTraceRandom :: forall g a . RandomGen g => g -> (forall s . IOSim s a) -> Trace a
```

**Are you willing to implement it?**
* [x] Are you? :smiley: