Threads and channels

Rust's ownership rules extend into concurrency. This theme covers thread::spawn, move closures, JoinHandle, message passing with channels, and shared state with Arc<Mutex<T>>. Planned exercises: 1. choose move for spawned threads. 2. recognise join. 3. choose a channel for message passing. 4. fill thread::spawn. 5. fill Arc::clone. 6. write a thread that returns a value. 7. write a channel sender and receiver. 8. translate TS async-looking work into threaded Rust. 9. fix shared mutable state with Arc<Mutex<_>>.