Should Outcome objects consume themselves when unwrapped?

[RazerM]

From python-trio/trio#466:

@njsmith:

It's generally a mistake to unwrap the same Result object twice, because if it's an exception you'll end up with a corrupted traceback as the same exception object gets raised in two unrelated call stacks.

We should at least document this, and maybe we should make unwrap "consume" the object so that if you call it twice then the second time it raises an error.

[smurfix]

Another possibility would be to call warnings.warn, but the PR documents my preference. ;-)