Miri should support concurrency

[RalfJung]

Miri currently does not support spawning threads. Would be nice if it did! This seems like a big project though.

Notice that this is about concurrency, not parallelism. So the Miri interpreter can remain single-threaded for this, it "just" has to support multiple interpreted threads each with their own stack, and then have a scheduler to pick which one to choose next.

Beyond the implementation effort, other issues that arise is: can we sometimes or even reliably detect UB due to data races? And how does Stacked Borrows fare with concurrency?

[RalfJung]

@christianpoveda @vakaras you both privately told me you were working on this. I figured I'd probably tell you both before too much work gets duplicated. ;)

[Aaron1011]

Can this issue be closed, or is there still more work to do?

[RalfJung]

We have A-concurrency Area: affects our concurrency (multi-thread) support for the remaining open issues around concurrency, so this one can likely be closed -- thanks.