Make use of [wrapping_]byte_{add,sub}

...replacing `.cast().wrapping_offset().cast()` & similar code.

library/std/src/lib.rs

@@ -296,6 +296,7 @@
 #![feature(panic_can_unwind)]
 #![feature(panic_info_message)]
 #![feature(panic_internals)]
+#![feature(pointer_byte_offsets)]
 #![feature(pointer_is_aligned)]
 #![feature(portable_simd)]
 #![feature(prelude_2024)]