sync: Make Lock more similar to std::sync::Mutex (#1573)

This renames `Lock` to `Mutex`, and brings the API more in line with `std::sync::Mutex`.

In partcular, locking now only takes `&self`, with the expectation that you place the `Mutex` in an `Arc` (or something similar) to share it between threads.

Fixes #1544.
Part of #1210.

tokio-sync/src/lib.rs

@@ -29,13 +29,13 @@ macro_rules! if_fuzz {
     }}
 }
 
-mod lock;
 mod loom;
 pub mod mpsc;
+mod mutex;
 pub mod oneshot;
 pub mod semaphore;
 mod task;
 pub mod watch;
 
-pub use lock::{Lock, LockGuard};
+pub use mutex::{Mutex, MutexGuard};
 pub use task::AtomicWaker;