Zero integer types

[kupiakos]

We've implemented some protocols that are designed to require all-zeroes in segments of the message. We ended up making this statically required by defining wrapper types around an enum. Example for u32:

#[derive(Clone, Copy, Default, IntoBytes, Immutable, FromZeros)]
#[repr(u32)]
enum AlignedZeroU32 {
    #[default]
    Zero = 0,
}

#[repr(C, packed)]
#[derive(Clone, Copy, Default, IntoBytes, FromZeros, Immutable)]
pub struct ZeroU32(AlignedZeroU32);
impl PartialEq for ZeroU32 {
    fn eq(&self, other: &ZeroU32) -> bool { true }
}

impl fmt::Debug for ZeroU32 {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        // No need to debug the inner field.
        fmt.debug_struct("ZeroU32").finish()
    }
}

// PartialEq against u32 with `== 0`, U32<T: zerocopy::byte_order::ByteOrder> with `== U32::ZERO`

There were enough hurdles to jump through to implement this correctly that it'd be convenient to have these as types within zerocopy. This doesn't use zerocopy::Unalign<AlignedZeroU32> because PartialEq and Debug require manual implementations