add experimental oneshot channel

The `oneshot` channel is gated under the `oneshot_channel` feature.

Author: connortsui20

library/std/src/sync/mod.rs

@@ -224,6 +224,8 @@ pub use self::poison::{MappedMutexGuard, MappedRwLockReadGuard, MappedRwLockWrit
 #[unstable(feature = "mpmc_channel", issue = "126840")]
 pub mod mpmc;
 pub mod mpsc;
+#[unstable(feature = "oneshot_channel", issue = "143674")]
+pub mod oneshot;
 
 #[unstable(feature = "sync_nonpoison", issue = "134645")]
 pub mod nonpoison;