Implementation: `#[feature(nonpoison_condvar)]`
Tracking Issue: https://github.com/rust-lang/rust/issues/134645
This PR continues the effort made in https://github.com/rust-lang/rust/pull/144022 by adding the implementation of `nonpoison::condvar`.
Many of the changes here are similar to the changes made to implement `nonpoison::mutex`.
There are two other changes here. The first is that the `Barrier` implementation is migrated to use the `nonpoison::Condvar` instead of the `poison` variant. The second (which might be subject to some discussion) is that `WaitTimeoutResult` is moved up to `mod.rs`, as both `condvar` variants need that type (and I do not know if there is a better place to put it now).
### Related PRs
- `nonpoison_rwlock` implementation: https://github.com/rust-lang/rust/pull/144648
- `nonpoison_once` implementation: https://github.com/rust-lang/rust/pull/144653
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