Re: the state of the concurrency branch

[Barry OReilly]

> Can you check it in?

Done.

> If there are multiple threads, the sleep may switch threads and
> cause the timer to run in a different dynamic environment.

It's a good point, though I think such code sounds dodgy.

Theoretically, it's already true on trunk that timers can run with
different dynamic bindings each time it runs. sit-for is called from
various places, timers can run within timers, and all can dynamically
bind variables.

If timers changed to each run in their own thread, dynamic bindings
would be easier to reason about. I don't know if that should happen
before or after merging to trunk (or not at all).