Fix theoretical UB while transmuting Arc (#588)

src/column_family.rs

@@ -71,7 +71,7 @@ impl UnboundColumnFamily {
     pub(crate) fn bound_column_family<'a>(self: Arc<Self>) -> Arc<BoundColumnFamily<'a>> {
         // SAFETY: the new BoundColumnFamily here just adding lifetime,
         // so that column family handle won't outlive db.
-        unsafe { std::mem::transmute(self) }
+        unsafe { Arc::from_raw(Arc::into_raw(self).cast()) }
     }
 }