Threadscope integration

[hyperthunk]

Not sure what this would look like but it was originally logged for -platform and I think if it has legs it's better suited here.

Presumably there's not much to do, as a process is implemented using forkIO already. Do we want to provide stats (such as issue #89 provides) there as well, or would it be better to expose them in some other bespoke tool?

[hyperthunk]

I don't think we need to do anything special here. I suppose we could associate process identifiers with thread ids, but most of what Threadscope does will just work, as a process is nothing more than a forkIO thread at heart.