add Bytes::is_unique (#643)

src/bytes_mut.rs

@@ -1708,6 +1708,7 @@ unsafe fn rebuild_vec(ptr: *mut u8, mut len: usize, mut cap: usize, off: usize)
 static SHARED_VTABLE: Vtable = Vtable {
     clone: shared_v_clone,
     to_vec: shared_v_to_vec,
+    is_unique: crate::bytes::shared_is_unique,
     drop: shared_v_drop,
 };