remove reference to value/error when unwrapping outcome

[graingert]

[TeamSpen210]

This might need to be a new method for backwards compatibility, since the attributes are public, users would be able to still read them after unwrapping. Unless we're fine with that breaking change.

[graingert]

could just call it InvalidatingOutcome, InvalidatingError and InvalidatingValue

[TeamSpen210]

A parallel class hierarchy seems problematic, they'd be incompatible with the current ones. More thinking like unwrap_and_clear(), that's all we need.

[CoolCat467]

Would using a weak reference not be acceptable?

[graingert]

Would using a weak reference not be acceptable?

I think that would be more complicated and it would be odd that unwrapped values are sometimes there sometimes not and not everything can be weakref'd

[RazerM]

I'm -1 on deleting public attributes during unwrap. I don't like that accessing an attribute/property may raise, but if we go that route then I'd much prefer AlreadyUsedError over AttributeError.

The way I've always used outcome is to only .unwrap() or .send(), so I would be fine with an outcome 2.0 that doesn't have the attributes, but I know Trio does use them.