True multithreading

[DJMcNab]

This is an extremely high level issue for tracking that we might want to be able to simulate multiple mir threads on multiple software threads sanely, for performance reasons, and centralising the discussion thereof.

The amount of work required for this would likely be quite large, so this is absolutely not a priority.

[bjorn3]

I honestly don't think miri will ever be able to do this without actually reducing performance. There is just too much ghost state that would need to be locked by multiple threads I think.