getting the current simulated time

I'm writing a program that needs to get the current time pretty often (it uses it to timestamp a [hybrid logical clock](https://cse.buffalo.edu/tech-reports/2014-04.pdf).)

Right now I can set delays, but I don't have a way to get the current time afterwards. My for-real `Effect -> Msg` handles it like this:

```elm
SaveAfter delay ->
Process.sleep delay
|> Task.andThen (\_ -> Time.now)
|> Task.perform Save
```

I see that I can advance time with `advanceTime`, but I'd love to be able to get at that counter in a simulated task!

Start by tracing how advanceTime updates simulated time and how simulated tasks handle Process.sleep. Compare that path with the Time.now call in the SaveAfter example. Done means a simulated task can read the current simulated time after time has been advanced, with behavior covered by the relevant tests.

Written by the indexing model from the issue text.