`DekuSize` does not include bytes from magic attribute

[hogg-sy-wang]

DekuSize is very useful to me, but I noticed an issue: the bytes occupied by the magic attribute are not being counted in SIZE_BYTES.

In some scenarios, I need to calculate the total number of bytes written to a file, which means the bytes from the magic attribute are always missing.

Here’s an example:

use deku::prelude::*;

#[derive(DekuSize)]
#[deku(magic = b"DEMO")]
struct Demo {
    foo: u32,
    bar: u32,
}

impl Demo {
    const SIZE: usize = Self::SIZE_BYTES.unwrap();
}

fn main() {
    let size = Demo::SIZE;
    assert_eq!(size, 12);
}

Running via cargo run:

thread 'main' panicked at src/main.rs:16:5:
assertion `left == right` failed
  left: 8
 right: 12
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Expected behavior:
SIZE_BYTES should include the 4 bytes from the magic attribute, so the total should be 12 instead of 8.

[wcampbell0x2a]

Same issue for seek* and pad_*.

[passcod]

Similarly, enums with no storage aren't sized properly.

Also, it doesn't work on structs that have a ZST field.