add support for deriving NoUninit on enums with fields

[Freax13]

We check for padding like we do for structs except that we also consider the enum discriminant when calculating the unpadded size.

[zachs18]

Suggested change

	_ => unreachable!(),
	_ => bail!("Deriving NoUninit is not currently supported for repr(Int) enums with fields")

#[repr(Int)] enums also have a (different) defined layout to #[repr(C)]/#[repr(C, Int)], so ideally those would also be allowed, but if you don't want to add that in this PR, then this should probably be a bail! with an error message instead of unreachable!.

E.g. this unreachable!() currently causes a "proc-macro derive panicked" for a #[repr(u8)] enum.

use bytemuck::NoUninit;

#[derive(Debug, Clone, Copy, NoUninit)]
#[repr(u8)]
enum Foo {
    A(u8),
}

error: proc-macro derive panicked
 --> src/lib.rs:3:30
  |
3 | #[derive(Debug, Clone, Copy, NoUninit)]
  |                              ^^^^^^^^
  |
  = help: message: internal error: entered unreachable code

[Freax13]

Good catch, thanks! AFAICT the layout is the same if there is no padding, so adding support for this was easy. I added a test for #[repr(Int)].