`parse_header` constructs an unaligned reference

[Freax13]

Running cargo +nightly miri test causes the following error:

Crash log

Preparing a sysroot for Miri (target: x86_64-unknown-linux-gnu)... done
  Finished test [unoptimized + debuginfo] target(s) in 0.01s
   Running unittests src/lib.rs (target/miri/x86_64-unknown-linux-gnu/debug/deps/xmas_elf-3090fb77e8659e43)

running 1 test
test test::interpret_class ... error: Undefined Behavior: constructing invalid value: encountered an unaligned reference (required 4 byte alignment but found 1)
 --> /home/freax13/.cargo/registry/src/github.com-1ecc6299db9ec823/zero-0.1.2/src/lib.rs:107:5
  |
107 |     mem::transmute(input as *const [u8] as *const u8 as *const T)
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered an unaligned reference (required 4 byte alignment but found 1)
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
  = note: BACKTRACE:
  = note: inside `zero::read_unsafe::<header::HeaderPt2_<u32>>` at /home/freax13/.cargo/registry/src/github.com-1ecc6299db9ec823/zero-0.1.2/src/lib.rs:107:5
  = note: inside `zero::read::<header::HeaderPt2_<u32>>` at /home/freax13/.cargo/registry/src/github.com-1ecc6299db9ec823/zero-0.1.2/src/lib.rs:42:9
note: inside `header::parse_header` at src/header.rs:27:17
 --> src/header.rs:27:17
  |
27  |                 read(&input[size_pt1..size_pt1 + mem::size_of::<HeaderPt2_<P32>>()]);
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `ElfFile::<'_>::new` at src/lib.rs:50:22
 --> src/lib.rs:50:22
  |
50  |         let header = header::parse_header(input)?;
  |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `test::interpret_class` at src/lib.rs:196:17
 --> src/lib.rs:196:17
  |
196 |         assert!(ElfFile::new(&mk_elf_header(1)).is_ok());
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside closure at src/lib.rs:194:5
 --> src/lib.rs:194:5
  |
193 |       #[test]
  |       ------- in this procedural macro expansion
194 | /     fn interpret_class() {
195 | |         assert!(ElfFile::new(&mk_elf_header(0)).is_err());
196 | |         assert!(ElfFile::new(&mk_elf_header(1)).is_ok());
197 | |         assert!(ElfFile::new(&mk_elf_header(2)).is_ok());
198 | |         assert!(ElfFile::new(&mk_elf_header(42u8)).is_err());
199 | |     }
  | |_____^
  = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error

error: test failed, to rerun pass `--lib`

Caused by:
process didn't exit successfully: `/home/freax13/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri runner /home/freax13/code/xmas-elf/target/miri/x86_64-unknown-linux-gnu/debug/deps/xmas_elf-3090fb77e8659e43` (exit status: 1)

The unaligned reference is created by the zero dependency. This dependency has already been patched to panic on unaligned references (nrc/zero@1d571c9), but there's no release containing this patch.

We probably want to prevent both UB and panics; I'm not sure what other parts of the code suffer from similar problems. Worst case scenario, we can probably put #[repr(packed)] on all the structs.