Could poisoning be implemented for Thunk to allow use with std::panic::catch_unwind?

[daboross]

If the closure passed into thunk::unsync::Thunk panics, then evaluate_thunk will leave self in the Cache { evaluating: () } state, while the flag is still set to Deferred.

This is partially correctly prohibited by Thunk containing an UnsafeCell and not implementing std::panic::RefUnwindSafe.

However, if thunk::unsync::Thunk implemented poisoning, then it could become RefUnwindSafe and be used in situations where std::panic::catch_unwind is useful. I think this would just look like setting the flag to some Flag::Poisoned directly before running the closure, right after line 168. Then if Poisoned is encountered, it could just panic with a message "thunk poisoned"?