Asserts in `Index` and `IndexMut` impls for `MemoryRegion` are slightly different

[WaffleLapkin]

Index:

assert_eq!(index as usize % mem::size_of::<usize>() as usize, 0);

IndexMut:

assert_eq!(index % mem::size_of::<usize>() as u32, 0);

That's a bit strange.