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Async from scratch 3: Pinned against the wall
So, we've covered polling. We've tackled sleeping (and waking). Going back to the definition, that leaves us with one core concept left to conquer: <em>pinning</em>!
Back in the ancient days of a-few-weeks-ago, I showed how we can translate any async fn into a state machine enum and a custom Future implementation. A Pin wraps a mutable reference of some kind (&mut T, Box<T>, and so on), but restricts us (in the safe API) to and replacing the value entirely, without the ability to move things out of it (or to mutate only parts of them). TrickOrTreat is safe and self-contained (as long as you only construct ::Init), but DemandTreat and PlayTrick are not, since they contain unmanaged raw pointers that could end up dangling.
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