add nonpoison::condvar implementation

Adds the equivalent `nonpoison` types to the `poison::condvar` module.
These types and implementations are gated under the `nonpoison_condvar`
feature gate.

Author: connortsui20

library/std/src/sync/nonpoison.rs

@@ -29,9 +29,12 @@ impl fmt::Display for WouldBlock {
     }
 }
 
+#[unstable(feature = "nonpoison_condvar", issue = "134645")]
+pub use self::condvar::{Condvar, WaitTimeoutResult};
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]
 pub use self::mutex::MappedMutexGuard;
 #[unstable(feature = "nonpoison_mutex", issue = "134645")]
 pub use self::mutex::{Mutex, MutexGuard};
 
+mod condvar;
 mod mutex;