Fix lies in the concurrency guide.

This cleans up blatant lies in the concurrency guide, and modernizes it
a bit. There's a lot more to do, but until I get to it, let's make it a
little bit better.
It would be great to get rid of all this exchange heap nonsense. Everything in a process is in the same address space, and Rust never attempted to do any isolation beyond that. It's also possible to transfer data without using dynamic allocations, since it's just doing the usual shallow copy.