Implement AsRef and Borrow for Arc (#325)

These traits are necessary to use the Arc in places that expect AsRef
and Borrow.

Signed-off-by: John Nunley <dev@notgull.net>

src/sync/arc.rs

@@ -1,5 +1,6 @@
 use crate::rt;
 
+use std::borrow::Borrow;
 use std::pin::Pin;
 use std::{mem, ops, ptr};
 
@@ -267,3 +268,15 @@ impl<T> From<T> for Arc<T> {
         Arc::new(t)
     }
 }
+
+impl<T: ?Sized> AsRef<T> for Arc<T> {
+    fn as_ref(&self) -> &T {
+        self
+    }
+}
+
+impl<T: ?Sized> Borrow<T> for Arc<T> {
+    fn borrow(&self) -> &T {
+        self
+    }
+}