Make scope() function soft-deprecated

Make crossbeam's `scope()` function soft-deprecated in favor of the more
efficient `std::thread::scope` that stabilized on Rust 1.63.

Author: taiki-e

crossbeam-utils/src/thread.rs

@@ -133,6 +133,8 @@ type SharedOption<T> = Arc<Mutex<Option<T>>>;
 /// returned containing errors from panicked threads. Note that if panics are implemented by
 /// aborting the process, no error is returned; see the notes of [std::panic::catch_unwind].
 ///
+/// **Note:** Since Rust 1.63, this function is soft-deprecated in favor of the more efficient [`std::thread::scope`].
+///
 /// # Examples
 ///
 /// ```