feat(shared): export Weak

Author: George-Miao

src/shared.rs

@@ -8,13 +8,13 @@
 /// [`Shared`]: sync::Shared
 pub mod sync {
     crate::cfg_loom! {
-        pub use std::sync::Arc as Shared;
+        pub use std::sync::{Arc as Shared, Weak};
     }
 }
 
 /// Singlethreaded [`Shared`] based on [`std::rc::Rc`]
 ///
 /// [`Shared`]: unsync::Shared
 pub mod unsync {
-    pub use std::rc::Rc as Shared;
+    pub use std::rc::{Rc as Shared, Weak};
 }