Miri preserves padding and partial initialization on copies

[RalfJung]

Miri should be able to detect that the following is UB because it prints uninitialized memory:

use std::mem;

#[repr(C)]
struct Pair(u8, u16);

fn main() { unsafe {
    let p: Pair = mem::transmute(0u32); // The copy when `Pair` is returned from `transmute` should destroy padding.
    let c = &p as *const _ as *const u8;
    println!("{}", *c.add(1)); // Print the padding byte.
} }

However, currently assignment is just implemented as an untyped memcpy, so we incorrectly preserve padding.

[RalfJung]

A related issue is that when copying, e.g., [usize; 3], Miri will not perform a "typed copy": a typed copy would make the entire array element "undefined" if any of its bytes is undefined, whereas Miri will just copy the underlying bytes. Also see this stackoverflow question.