Fix (and improve) enum representation of an async fn future

[danielhenrymantilla]

g: MutexGuard<'_, u32> does not exist in the scope of the "unresumed" generator / future, so this fixes it.

On top of that, I have taken the freedom to add a the Return variant to the enum, and have also added some comments on top of it, in order to have a precise recap' of the cited post about generators.

[rust-highfive]

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[alexcrichton]

r? @davidtwco

[davidtwco]

Thanks for the PR! I've left a few comments below.

[davidtwco]

@danielhenrymantilla thanks!

@alexcrichton can you merge this? I'm not sure I have permissions for this repository.

[alexcrichton]

👍