Pin Cb7 field layout

Cb7 casts seeds through bytemuck as u32 words. repr(C) keeps seeds at
offset 0. The u32 key field keeps the struct aligned to 4 bytes, so
the cast remains aligned even if Rust layout choices change.

Author: mlafeldt

src/cb7.rs

@@ -7,6 +7,7 @@ use core::fmt;
 use bytemuck::{bytes_of, bytes_of_mut, cast_slice};
 
 /// A processor for CB v7+ codes.
+#[repr(C)]
 #[derive(Clone)]
 pub struct Cb7 {
     seeds: [[u8; 256]; 5],