Add missing #![deny(unsafe_op_in_unsafe_fn)] to upb/rust/lib.rs and fix violating functions

PiperOrigin-RevId: 655226304
diff --git a/rust/upb/lib.rs b/rust/upb/lib.rs
index bbfc106..650baec 100644
--- a/rust/upb/lib.rs
+++ b/rust/upb/lib.rs
@@ -4,8 +4,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
+#![deny(unsafe_op_in_unsafe_fn)]
 
 mod arena;
+
 pub use arena::{upb_Arena, Arena, RawArena};
 
 mod array;