Consider ways to simplify the variants for Promise's outputs

[mxgrey]

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Description

Promise uses PromiseState as its output for both sync and async usage.

Because of the many ways a request could become unable to deliver, there are several more variants than a user might expect. We should consider ways to simplify this so users aren't left scratching their heads over how to handle variants that they can't receive anyway.

• We return PromiseState as the output of .await, but it contains a Pending variant which can't be the case after .await is finished.
• We use Promise as the output of Channel::command, but that method will never result in a Cancellation and shouldn't realistically result in a Disposed either.
• Taken is a variant we need to have because we store the result in the Promise, and a user can take it without dropping the promise. The existence of this variant might confuse users because they don't know what to do if they get it.