temp and assert_eq

[carlocorradini]

Allow temp to be combined with assert_eq.
If both attributes are combined, a build error is generated because the field cannot be found in the scope.

#[deku_derive(DekuRead, DekuWrite)]
pub struct Packet {
    #[deku(temp, temp_value = "Self::SIZE", assert_eq = "Self::SIZE")]
    size: u16,
    // ...
}

impl Packet {
    const SIZE: u16 = 128;
}

In this scenario, the packet size is known, hence there is no need for a field in the final struct. However, when reading, I want to ensure that the temporary size matches my known value. When writing, keep the same known value.