transmute_copy doc example is too trivial

[tomjakubowski]

http://doc.rust-lang.org/nightly/std/mem/fn.transmute_copy.html#examples

use std::mem;

let one = unsafe { mem::transmute_copy(&1) };

assert_eq!(1, one);

This transmutes an i32 to... an i32! A more realistic example might be to transmute a byte array to a packed struct.

[steveklabnik]

part of #29362

[frewsxcv]

I took a stab at this: #31720