Update unsafe trait example to zerocopy version 0.8 (google#2434)

Zerocopy crate version 0.8 introduced changes to its API, which caused
the example code to break.
google/zerocopy#1680
> AsBytes ->
[IntoBytes](https://docs.rs/zerocopy/0.8.*/zerocopy/trait.IntoBytes.html)

---------

Co-authored-by: Martin Geisler <[email protected]>

src/unsafe-rust/unsafe-traits.md

@@ -8,36 +8,31 @@ Like with functions, you can mark a trait as `unsafe` if the implementation must
 guarantee particular conditions to avoid undefined behaviour.
 
 For example, the `zerocopy` crate has an unsafe trait that looks
-[something like this](https://docs.rs/zerocopy/latest/zerocopy/trait.AsBytes.html):
+[something like this](https://docs.rs/zerocopy/latest/zerocopy/trait.IntoBytes.html):
 
 ```rust,editable
-use std::mem::size_of_val;
-use std::slice;
+use std::{mem, slice};
 
 /// ...
 /// # Safety
 /// The type must have a defined representation and no padding.
-pub unsafe trait AsBytes {
+pub unsafe trait IntoBytes {
     fn as_bytes(&self) -> &[u8] {
-        unsafe {
-            slice::from_raw_parts(
-                self as *const Self as *const u8,
-                size_of_val(self),
-            )
-        }
+        let len = mem::size_of_val(self);
+        unsafe { slice::from_raw_parts((&raw const self).cast::<u8>(), len) }
     }
 }
 
 // SAFETY: `u32` has a defined representation and no padding.
-unsafe impl AsBytes for u32 {}
+unsafe impl IntoBytes for u32 {}
 ```
 
 <details>
 
 There should be a `# Safety` section on the Rustdoc for the trait explaining the
 requirements for the trait to be safely implemented.
 
-The actual safety section for `AsBytes` is rather longer and more complicated.
+The actual safety section for `IntoBytes` is rather longer and more complicated.
 
 The built-in `Send` and `Sync` traits are unsafe.