sync: adds Notify for basic task notification (#2210)

`Notify` provides a synchronization primitive similar to thread park /
unpark, except for tasks.

tokio/src/loom/std/mod.rs

@@ -64,6 +64,7 @@ pub(crate) mod sync {
         pub(crate) use crate::loom::std::atomic_u64::AtomicU64;
         pub(crate) use crate::loom::std::atomic_usize::AtomicUsize;
 
+        pub(crate) use std::sync::atomic::AtomicU8;
         pub(crate) use std::sync::atomic::{fence, AtomicPtr};
         pub(crate) use std::sync::atomic::{spin_loop_hint, AtomicBool};
     }

tokio/src/sync/mod.rs

@@ -406,9 +406,18 @@
 //! * [`Mutex`][Mutex] Mutual Exclusion mechanism, which ensures that at most
 //!   one thread at a time is able to access some data.
 //!
+//! * [`Notify`][Notify] Basic task notification. `Notify` supports notifying a
+//!   receiving task without sending data. In this case, the task wakes up and
+//!   resumes processing.
+//!
 //! * [`RwLock`][RwLock] Provides a mutual exclusion mechanism which allows
 //!   multiple readers at the same time, while allowing only one writer at a
 //!   time. In some cases, this can be more efficient than a mutex.
+//!
+//! * [`Semaphore`][Semaphore] Limits the amount of concurrency. A semaphore
+//!   holds a number of permits, which tasks may request in order to enter a
+//!   critical section. Semaphores are useful for implementing limiting of
+//!   bounding of any kind.
 
 cfg_sync! {
     mod barrier;
@@ -421,6 +430,9 @@ cfg_sync! {
     mod mutex;
     pub use mutex::{Mutex, MutexGuard};
 
+    mod notify;
+    pub use notify::Notify;
+
     pub mod oneshot;
 
     pub(crate) mod semaphore_ll;