Added Arc::try_pin

This helper is in line with other other allocation helpers on Arc.

Author: alex

library/alloc/src/sync.rs

@@ -489,6 +489,13 @@ impl<T> Arc<T> {
     pub fn pin(data: T) -> Pin<Arc<T>> {
         unsafe { Pin::new_unchecked(Arc::new(data)) }
     }
+    
+    /// Constructs a new `Pin<Arc<T>>, return an error if allocation fails.
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[inline]
+    pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
+        unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
+    }
 
     /// Constructs a new `Arc<T>`, returning an error if allocation fails.
     ///