tokio-sync: Add async mutual exclusion primitive (#964)

This PR introduces `Lock`: A concurrency primitive built on top of `Semaphore` that provides a `Mutex`-like primitive that interacts nicely with futures. Specifically, `LockGuard` (in contrast to `MutexGuard`) does _not_ borrow the `Lock`, and can thus be passed into a future where it will later be unlocked.

This replaces #958, which attempted to introduce a less generic version. The primitive proposed there will instead live in [`async-lease`](https://github.com/jonhoo/async-lease).

tokio-sync/src/lib.rs

@@ -24,6 +24,7 @@ macro_rules! if_fuzz {
     }}
 }
 
+pub mod lock;
 mod loom;
 pub mod mpsc;
 pub mod oneshot;